First of all, THANK YOU for being so honest!!! You are NOT the worst mom in the world!! I knew I wasn't the only one that had days like this, but I'm so ashamed by these moments that I've never confided in anyone, so THANK YOU!!!
Secondly, I don't think this really had anything to do with the breastfeeding aspect; it probably has more to do with the emotions going through you right now. You're stressed because of returning to work and trying to make everything flow smoothly (milk included). You probably feel a little guilty for having to leave her so young, and you're also suffering a little from not being able to see your baby all day every day :( I'm fortunate enough to be a SAHM with my 2 kids (3.5y, 6mo), and I honestly couldn't imagine how I would feel if I had to return to work, especially at 4mo. So, cut yourself some slack with all these changes....
As a peer counselor with Nursing Mothers Counsel, she does NOT sound like she's ready to give up nursing. VERY few babies wean themselves at 4mo, so take heart and have faith! Your daughter has been through a lot of changes, and so have you, in the last week! If you want to continue nursing, great. If not, that's fine, too, but it sounds like you're committed to breastfeeding, and that's awesome! That being said, is she nursing well on your lunch break? If so, you more than likely have nothing to worry about. One day is not going to make or break your supply, so you don't need to stress about that either. What makes you believe you haven't had a good milk supply? The main thing that influences your supply is stimulation, so if she's not nursing, you may need to hand express or pump (if you have one) to help maintain, especially if this continues through the weekend. I'm not sure about your work routine, but to maintain your supply through the workweek, you'll need to pump whenever your daughter would nurse but can't (or won't).
We also need to remember that babies take more from the bottle because they have no choice. Gravity rules when it comes to bottles because the milk/formula just drips out when it's upside down, and this is the traditional way to bottle-feed a baby. To help facilitate breastfeeding through the bottle, we should sit our babies up on our lap (like they're sitting at the dinner table) and hold the bottle (parallel to the dinner table) and have the milk just covering the opening of the bottle, so she has to work to get the milk out just like she has to work at the breast. She may be experiencing a little bit of nipple preference right now, but if you try this method of bottle-feeding, you may find she goes back and forth more seamlessly.
How was she acting today before you gave her the bottle? Was she happy? Fussy? Acting hungry? Her behavior pre-bottle will help you understand why she was refusing the breast. Honestly, she may just not have been hungry. As parents, sometimes we think our babies should eat the same amount at the same time day after day, but this just isn't true. We need to understand that our babies have appetites just like an adult does; their needs fluctuate day to day and even throughout the day. She's also at an age developmentally where she's able to go longer without feeding because she takes in more per feeding than she did even a week ago. Everything about your baby is going to change SO FAST in the first year that the only you can count on is that it's going to change :) You may also be noticing that the cues she gave you a week ago that said she was hungry (sucking on her fingers or fist, licking her lips, tonguing objects, etc) may not necessarily mean the same thing this week.
Try not to be so hard on yourself; EVERY mom has days that they're going to feel guilty about. And sometimes as much as we tell ourselves we're never going to do "that" again, we may find ourselves in the indelible position that we just did "that" again!! We're human, and no human is perfect, so please don't beat yourself up over this :( That your daughter was laughing and playing so soon after the incident should show you that our kids are more resilient than we give them credit for. The best thing you can do to make up to her is to walk over and LOVE her. Snuggle her, kiss her, apologize to her, tell her how much you love her, and just let her know through your actions that you still love her!! She'll probably act like nothing even happened....
Hope this helps, but if you want more information about breastfeeding, please feel free to visit our website at http://www.nursingmothers.org, or you can email me off-list at ____@____.com are a non-profit organization who provides free one-on-one education and support to breastfeeding moms at all stages mainly through phone counseling, but we also do free home visits and teach breastfeeding classes to pregnant moms. Try to let it go, and have a great weekend!