It's hard to properly imagine an uncomfortable, aggravating biological condition that affects organs you simply don't have. It's probably as hard for your average guy to imagine what it would be like to menstruate as it is for the average gal to imagine what it's like to suffer from a fractured penis.
Pain is part of the human condition, and we can all relate to plain ol' pain. It's the particulars that get real fuzzy real quick, especially for something that creates such a complicated and variable set of symptoms as menstruation.
So. We'll have to use the organs at hand for this descriptive exercise. If you have a penis, and want to know what menstruation might be like for your girlfriend, sister, or mom, read on!
Start by imagining that your urethra is quite a bit larger than it is now. Now, imagine that you have a magical prostate gland that holds back urine but does nothing to hold back blood and tissue.
Yes, that's right, boys, you're going to be bleeding through your dick for the next several days! This is fun already, isn't it?
Now, imagine that, overnight, a mass roughly the size of a ping-pong ball or a hen's egg has grown inside your bladder. This mass is free-floating, and has a hard surface much like that of a cheese grater. On the third day or so, your hormones will work another feat of magic and the mass will rapidly shrink down to a size you can easily pass.
Because this mass has taken up 1/4 to 1/2 the normal volume of your bladder, you have to pee more often than usual. Sometimes, a lot more than usual. And while it's bouncing around in there, it starts to grate off the inner lining of your bladder. Painful!
So when you're not having to run to the bathroom to pee, you're bleeding. You have to wear a pad, sometimes two if you're bleeding quite a lot. They chafe the inside of your thighs and your balls, and sometimes your pubic hair gets caught in the adhesive backing.
You decide that pads suck, so you stick a cotton wad in your urethra to stop the blood. It can chafe quite a lot if there's not much blood flowing when you put it in, and if often chafes coming back out if you have to remove it to pee.
If you're lucky, you can't feel the wad in there, even if you get an erection, but if you have a smaller penis, you almost always feel it. It doesn't hurt exactly, but when you sit down you're aware that you've got a foreign object lodged in your dick, and it's not an awesome sensation. Also, it seems to make the cramping from the little landmine in your bladder worse.
And when you pull it out, there's sometimes a lovely little backlog of tissue in there. Clots of blood and reamed-off bladder lining come slithering out of you like warm slugs. In that moment, you so love your body, and just feel ever-so-sexy.
Your girlfriend, if she deals well with blood, is quite keen to have sex with you, since you're infertile while all this bleeding is going on. Otherwise, she's avoiding intimate contact with you on the grounds that you smell weird or you'll get blood all over her sheets. If you're especially unlucky, your girlfriend will be totally unsympathetic to your situation: You go through this every month, John, I'd have thought you'd have learned to deal with it by now. It's only a little pain, go take some Advil and be a man about it!
Meanwhile, you feel run-down and mostly want to sleep, the inside of your dick is chafed, two pairs of your drawers are stained with blood, the inside of one of your internal organs is peeling off, and sometimes the pain meds just don't do the job.
And that, my friends, is what it can be like to menstruate.