What does it mean if you have a wet stain on your underwear (not blood) when you have your period?

so i have my period right now, and it's pretty heavy. i went to the bathroom because I thought my tampon was leaking as I felt "wet" down there, and it turns out there was no leakage of blood, but there was a watery like leakage, maybe pee? maybe something else? I dont know but i'm only 26 so i shouldnt be peeing my pants. Help! What else could this be?