I am 31 weeks pregnant & I had mild ulcerative collitis and was on salofalk granules 3gms/day. Now I start getting bloody/mucus stool, do I start med?

Call MDs. Don't start any meds without checking with your ob. Gut symptoms? See your gastroenterologist to see what is going on inside-could be hemorrhoids, or not, or more than that. Not uncommon for uc to flare in the 3rd trimester, especially if meds were discontinued.
Yes. The most important thing is your health and getting your uc under control. It is safe to stay on Mesalamine products as well as start steroids or even consider a biologic agent. Go see your doctor.