Horrible back pain at 30 Weeks?

Okay so I know this is going to sound weird, but I only have back pain after I've laid down in bed right after work in the morning (I work night shift). I don't have a strenuous job, in fact I sit all 8 hours besides a single 15 min round around the factory. My back doesn't hurt at all during the night while I'm sitting or doing my round or once I've gotten home. But once I lay down for a few minutes and then get up to pee (baby on the bladder yyyaaayyyy) the pain in my lower back is so bad once I roll out of bed that I have to clench my teeth so I don't start cussing really loud from it and can't take anything larger than ity bity shuffling microsteps were I don't lift my feet because I physically can't move ANY part of my body without making the pain radiate 10x over. Now if this happened every time I got up I'd understand ya know? Like, "Oh I must be sleeping funny or not propped properly for the baby"! But no. Once I get to the bathroom and finish 95% of the pain has eased away and once I go back to bed and get back up later in the day there is absolutely zero back pain no matter how often I get up or how I was sleeping. It only started this past like week and a half and I just don't get it! I haven't heard of anyone else having this problem. Is this normal or should I ask my OB? Would my OB even know what would be causing it??