This happened when I was pregnant with my second child. My older son got roseola (not fun and I feel for you taking care of your three year old during the couse of the virus---- the rash stage being very hard). I would absolutley tell your ob/gyn. The risk is minimal but if you are not immune, they have more of a concern. Problems that have been associated are in women much farther along in pregnancy.
But I too remember being worried. My doctor was reassuring and yours probably will be too----- so tell them at your next appt. or if that is not for another month, then give the office a call just for peace of mind.
Good luck and hope your three year old is all better. Happy pregnancy!
Since almost all adults are immune ,there is no known risk if you are pregnant.Most people have antibodies by the time they are 4 years old .Good luck