Actually, it might just be normal.

When you organism, you actually experience uterine contractions. A small organism can produce 3-5 contractions and a big organism (!) can produce up to 15 contractions. These are usually quite powerful. (Imagine as these they are the same type that allow the birth of a child!)

Afterwards, you could feel pain like muscle strains.

There is a tremendous amount of sexual tension that occurs in your body, pelvis, thighs, back, etc causing strain in various parts. If you don't organism, it could lead to more muscle strain because the energy is not released.
But organism alone can produce "strain" afterward.