This sounds very much like what I had with my new baby. For me, I noticed it the day after my baby was born. It may have been there sooner, but I just had not looked much at my stomach, etc. Anyway, I had a rash all over my stomach, going down my legs, and a little on my breasts. It was somewhat itchy but not too bad. My midwife looked at it 6 days postpartum. She couldn't figure out what it was. She had not seen this before, and she has done LOTS of births. She said it was *not* an allergic reaction to anything, including antibiotics. She said it would go away, and it did after about 3 weeks.
I have GBS and had 1 bag of penicillin in labor and a little IV fluid with that. After birth, I had a little pitocin. I ended up with LOTS of swelling, which is something I did not have with my first two babies - even though I had much more intervention with my first two.