Everything you describe sounds fairly common. I agree that the pain is likely related to the stretching of the round ligaments and/or the relaxing of the pelvic structure.
The rash is likely the pupps rash that a previous poster mentioned. For most women, it only goes away once the baby is born. Many people find relief using oatmeal-based lotions and baths. Aveeno is a good brand for those products.
As for the antibiotics, I am extremely curious why any doctor would prescribe them for a cold, which is a viral infection, not the result of a bacterial infection. The antibiotics can exacerbate the rash because they indiscriminately remove all bacteria from your body, even the good ones. You may want to replenish your good bacteria by taking probiotics (available at health stores) or eating lots of yogurt.
Hope this helps!