Is my girlfriend cheating? if so what do i do

Alright guys i have an issue.
im in a long distance relationship with my gf. i see her at least once sometimes twice a month. in december we'll be celebrating our year anniversary. she lives with my best mate in uni accomadation

now here lies the problem.

we split up about 4 weeks back, got back together 2 weeks later.

my best mate came home and we had a few drinks he said he had to tell me something.
on halloween (the anniversary of when we first met) she went out on a works drink do. she let a male friend/collegue as the taxi back to his wouldcos a bomb and theres regular buses in the morning. they came home, got high and went upstairs
now another one of her housemates (who slept with said colleague on a previous occasion) spreading rumours that she heard sex noises coming from her bead room. no one else claims of heard anything..this was right before she split on me, all be it temporarily. she told my best friend about these rumours before he heard to denie it
i spoke about it on the phone to her, she was pretty kl and knew where i was going before i finished my first statement and said admitted a male friend had stayed on her floor (never known her to have a spare mattress but hey) she said the other housemate was spreading jelly to be the centre of attention as usual.
she got a bit funny and said "god forbid i have a friend, im bi-sexual so even if it was a girl this would still be going around".
my gut is telling me to believe her
however still not 100% convinced
otherwise we're stronger than ever, seeing her this week for a long weekend an dd im meeting her parents for the first time.
i want to avoid questioning her further because i dont want to push her away, especially if shes innocent and her girly housemates will protect her no matter what
so how do i get to the bottom of this? with proof one way or another
and if its bad, how do we get past this
i know she loves me, and i love her, thats without question on both sides

Just dump her and keep her dumped this time. You don't trust her and that will drive you nuts when you're not with her, which is most of the time.

Times be a changing. Seems in younger generations many don't think twice about having a chum sleep over, be them in another room, the same room or the same bed. Metro sexualism is in full on mode these days and while it may seem kosher to have sleep over's with same or opposite gender buddies, we are still humans here. We do get protective now don't we.
You need to follow your gut here. Listen to your inner spidey sense and see the truth on her face, body language when you ask her if anything happened.
Personally, I think of myself as open minded but I would not like my sweetheart sleeping in the same room with anyone other than me, be them on the floor or not, I wouldn't care. And more so, my sweets would never do that to me nor I he.
Your lady needs to learn about respecting boundaries and not just hers but yours.
But I agree with Wakeup mostly here; without trust, there's no foundation to grow from.
good luck