Not sure what it could b!

Nikki • I`m 30 engaged to the love of my life! Pregnant with our 1st child!!
I've been for the last couple months been having lower stomach pressure and pain, once it gets bad I relax and seems to go away. Dr says it cuz baby is very low! But tonight the pains r a lil more then they have been n pressure too! No matter what way I sit, stand or lay it's there! I'm 31 weeks and 4 days! Does anyone know what it could be??