Issues walking... help!

I'm currently 27w1d with my first baby and I had no idea what I would be in store for. My OB tells me to keep exercising and such so my boyfriend and I go on a walk 3-4 times a day. Recently, my lower back will hurt real bad and like it feels like under my stomach is tearing/aching after like 15-20 minutes.

This just started happening and I'm kind of confused. Is this round ligament pain or something else?