Hey! I have a 5-month old baby girl, born Dec. 17. I totally breastfeed so in the beginning when she wouldn't have a bowel movement for a few days, I was concerned. But I did read that as they get older they may go a few days w/o bowel movements so this eased my mind and in your case your baby will probably have more of a problem since you've introduced a little formula b/c it can constipate them. But I wouldn't worry too much about it unless she goes a really long time in between movements and this happens a lot.
Something that helped my baby is taking her to the CHIROPRACTOR. My sister has also taken her baby girl, who was constipated as well - her baby had a bowel movement the same day - worked great for her. Her adjustment also helped her baby sleep better at night :) I've been taking my baby to the chiropractor since she was 3 weeks old. Her first couple of visits, the doc. could tell she was having some problems in her intestine area. He adjusted her and she was fine. I've noticed that each time I take her it seems to help her go to the bathroom better & it helps with her gassiness too. So, you may want to give this a try. Also, my sister-n-law was told by her lactation consultant to massage her baby's booty area to help her have a bowel movement - it worked.
I saw where you posted that things are going better now since you've increased your fiber & water intake. That's great!! In my opinion, I wouldn't give her the karo syrup - I would go for a more natural approach, but that's just me. See what your doc. says though. Give the chiropractor a try - it's completely safe and the results are great! Good luck!!