I have this rash of small red itchy bumps spread out. For 5 days. On upper back, arms, inner thigh, sides. 39wks preg. What should I do? What is it

Take a medication. The rash that you describe can be due to many causes. At this point you can take an otc class b antihistamine such as Cetirizine 10mg and apply an anti-itch cream such as sarna with pramoxime (not benadryl). If no better or worsening, see you physician.