Help.......

18 weeks pregnant with rainbow baby here...

Since about 14 weeks when I getup at night to go pee I get this horrible pain in my back AFTER I relieve myself. I make sure I'm not straining or anything. It goes from lower spine to upper spine and is not like normal sharp back pain, it's a dull pain that makes it's so I can't stand up, like after you've been leaning in tour wrist too long and it's hard to move it .. anyone have any idea of what this could be?