Is my gf cheating on me?

why they hell

Investigate further is what I say. Has she had any odd excuses for spending time away? Slipped up on anything in past conversations? If you want to know the truth, you'll need to wait for more info from her, or press the issue yourself. If you wanted to give the cuckolding scene a try, try maybe casually joking with her on the subject.

Perhaps:

"You really think it's that easy to hide? I dare you to come home wet and try to full me."

or even possibly a straight confrontation but with an upbeat and open mindset. Ask her if she's ever though of being with another guy or possibly a threesome. If she bites, tell her it's been one of your fantasies for a while and that you wouldn't mind her exploring with another guy, as long as she keeps you completely involved and has your consent before doing it.

Good luck friend. I hope to hear more from you.

yea thats crazy

So from the last thing you wrote I'm assuming you're not wanting to do the cuckolding scene? Correct me if I'm wrong.

From what you've said, it does seem to somewhat sound like she may be talking to someone else. Especially since she's into the threesome. The whole "sick" excuse sounds like a cop out to me.