What can this be?

Maria

Hi!! So I masturbated yesterday but I didn’t put anything inside my vagina. Today I woke up and after peeing I saw this on the toilet paper. I haven’t had sex in about a month and I recently got tasted for all STIs and I’m clean. I have no other symptoms to go with this kind of discharge. I had my period about a week ago, could it just be some old blood that was there and came out after I masturbated?