I'd go with the advice of your OB/GYN - they're the experts. However, I also researched the active ingredients, and you are correct, with the exception of the actual acetominophen, they all appear to be Pregnancy Category C meaning they either haven't been studied or the information hasn't been submitted to the FDA for approval for that population. It's up to the discretion of your OB to determine (in concert with you) if the benefits outweigh the risks).
If you're not comfortable, call your OB back and ask for other solutions. The saline suggestion is a good one - particularly neti pots. If it doesn't work, you know you need something else.
Are you sure it's a cold vs. a sinus infection? You may want to have your Primary Care Provider assess if you have an infection because it will not improve without an antibiotic. I have a sinus infection right now, and it took drainage from my eyes before I went to the doctor and got an Augmentin Rx (which is safe during pregnancy).