Hey, Anon. So sorry you're dealing with this.
How long have you been feeling this pain? Has it been an ongoing thing for some time now, or did it develop recently?
(And if the answer is that it developed recently, can you think of anything else that might have changed around that time?)
I actually had pain very similar to yours for a while, but I work at a computer all day and so I sit much more than you do. However, the symptoms sound the same, and the area of pain sounds the same.
Have you ever heard of the sciatic nerve, or sciatica?
Here's an illustration of where that nerve runs:
The sciatic nerve actually begins in your lower back, moves down through your buttocks and down the back of your leg. So the problem can actually start in your back and not necessarily your leg. Also, the pain can stop where yours does or travel further down the leg as well.
Do you have lower back pain? (I'm guessing that after carrying three babies around, it's most likely you do!) Do you ever have tingly sensations or numbness? It's possible that you have a compressed or herniated disc or a pinched nerve that's giving you trouble. When you take the Advil, does it help?
Here's our EmpowHer encyclopedia page on Sciatica, its causes, symptoms and treatments:
Does this sound like it could be what's bothering you?