according to my own doctors when it has happened to me...

if it only happens after sex/masturbating it is nothing to worry about. the muscles being worked out during this activity do not usually get a work out otherwise, so sometimes they can ache and cramp afterwords. to put it in a different light. the muscles used for sex and masturbation are the same muscles needed and active when in labor, the pain is the muscles contracting, squeezing. they get tired or over used you get pain, just like you would in your legs if you ran a mile before stretching and havent run a mile in a long time.

solution not to do marathon sessions. take it slow and easy, its not a race to the finish line. slower, less frantic, is better. take time to enjoy instead of wham bam thank you ma'am. (my doctor loves cliches lol)

other tips dont insert so far. the upper portion of the vagina and the cervix / womb do not have nerves that tell you when you are doing something potentially harmful unless its like a quick forceful jab. inserting too far, too fast and not using lube can bruise a womans insides.

make sure your toys, hands what have you are clean and is a comfortable fit for your body.

the need for bathrooming afterwards is normal. its natures way of flushing out the urinary tract of harmful bacteria that may have entered the urinary tract.

some women when they have reached their orgasm, they dont always to put it bluntly and what its called "squirt" instead the orgasm fluid remains in the bladder until you go into the bathroom. sometimes when the bladder is full or over full there is a pain that tells a person they need to go to the bathroom. my suggestion go to the bathroom before you do your thing so that your bladder doesnt get so full of fluid during the high point of your activity.