Hello ?? It's week 21 of pregnancy and today is my second day with low…

Hello ?? It's week 21 of pregnancy and today is my second day with lower back pain. I can't sit down anymore and on top of that, I have weakness all morning (and feel a bit nauseous), even though I'm just lying at home doing nothing. I don't know why this might be happening (as it's my first pregnancy). Do you know what could be causing this? Should I go see a doctor?