The most likely diagnosis is a virus of some sort. When you're pregnant these can actually occur more frequently since you're somewhat immunocompromised (immune system isn't in tip top shape). That said it can also become a bacterial infection more quickly because of this as well.

I'd recommend plenty of fluids and rest. Avoid medications if possible but many otc products are safe at this point in pregnancy. If you aren't getting better in several days or if you get worse then it's worth being seen.

Sumeet K. Goel