Bit of an embarrassing problem but I had sex on a beach on quite hard sand and now my lower back in the middle slightly to the left feels very sore and bruised. It hurts even when I sit up in bed with a pillow behind it, whenever there is slight pressure on it it's very painful this is why I think it seems to be more likely a bruise rather than muscle spasm etc because it doesn't hurt too much if I'm just walking around, but does still hurt abit if I roll over in bed etc.