Uh, go see your doctor.
It's a bit of a crap shoot without examination.
The fact that it's improving is a good sign. Most "injuries" (quotes because it's most likely just a muscle spasm) that occur like that are "brain farts" where one muscle just doesn't fire like it normally would and a spinal segment will shift and BANG! Muscle spasm.
If you find laying on the floor with your legs supported up on a chair (hips and knees at 90 degree angles) has a relieving quality to it, your psoas may be in spasm (the standing up thing is a clue as well).
Keep icing off and on for another 24 hours and see if it calms down some more. Then search for a hip flexor stretch at exrx.com and begin gentle stretches.
Bill, Bill, Bill, Bill......