Tummy troubles

My 7.5 week DD started having tummy problems yesterday - fussing, straining and struggling, and making her little body and tense and stiff - I guess it lasted most of the day and then got better after she pooped, then started up again a while later. She slept ok last night but around 2 passed gas for like 5 minutes straight! She had 2 BM's in the night. Then this morning she started struggling in her sleep and when she finally woke up was fussy. She slept most of the morning, and my mom (our babysitter) called to tell me she's still having trouble this afternoon.

I just started back at work part-time last week, and haven't been able to pump quite enough to fill in her feeding schedule, and don't have enough milk stored up to give her breastmilk exclusively, so we started supplementing with formula. We had to supplement her with it when she was first born (hydration issues with medication) and she had no problem with it then. She takes it ok - the bottle seems more off-putting to her - but when I called the nurse she seemed to think it was probably the formula (similac advance with iron) and suggested that (after trying gas drops) we switch to soy formula.

So I was wondering if that sounds right to you all? ANd if the way we prepare formula would make any difference (we've been using bottled "baby water" and not heating it up before mixing it - the powdered kind)? I hope to get back to pumping enough so that she eats breastmilk exclusively but I don't know how realistic that is with my schedule. It may also be possible she has a cold - we were around someone who had one, oops - could that affect her digestive system?

Aarrggh, this is the first time she's been sick or really fussy and I just feel so bad for her, and I miss her smiling little cheerful usual self.