It really is normal for kids your daughter's age to regress after the birth of a new baby, and it sounds like this is what your daughter is doing. With any luck, preschool is just about done for the year?
I would put her in Pull-Up's for preschool so that her classmates don't have to know about the "accidents." At home though...go back to potty training mode. That is, put her in a dress with nothing under it. I found that my kids were better at using the potty in the early stages if they had no underpants on. Regardless of whether she is wearing underpants, a pull up, or nothing; when she does have an accident, make it her responsibility to clean up, and try not to act upset with her--give her as little attention regarding the issue as possible because the bottom line is that this behavior is something she is doing (unconsciously) to get attention--so if you want it to stop, you have to stop rewarding it with attention.
Obviously she needs help cleaning up a poopy mess--but try not to make a big deal about it. She made a mess, now she needs to clean it up--just as if she spilled her drink at a meal. Also, when my second DD and again my first son had trouble with potty training with #2, we just started throwing away underpants that were really messy--too much effort to clean. But the deal was, it was *their* problem, *they* had to fix it. Which meant they had to use their own allowance money to buy new underpants. At just $.50 per week allowance...well they each bought just one pack of new underpants and realized pretty quickly that they'd rather be spending their allowance on toys. (we do $.50 per week at age 3, $1 at age 4, $2 at 5, $4 at 6, and then start raising just a dollar a year...although now our oldest DD is to the point that we intend to transfer over to her the responsibility for buying her school clothes, so she's going to get a spike in her allowance).
Try to spend some special time with her every day--make it part of the routine so that she can expect it. Don't stress about the potty issues--just have fun with her. As frustrating as it is now (I've dealt with similar issues with 3 out of 4 of my kids), she will grow out of it.
Good luck!