It don't have to be an std, it can be BV it have a fishy smell or a bad case of yeast infection.
Semen can have a bitter or bleachy smell sometimes. That's normal. However if you are getting any kind of "rotting fish" type smell, that can be a sign of an STD and should be checked.
semen does smell, if you are really worried about it then get it checked out.
The blood strings during period is normal you are just having a heavy period. The brownish is considered old blood and it happens toward the end of your period or when you are flowing ligter usually. What your talking about after sex is probably the semen from sex.