I'm not one to normally say you need to ditch your girlfriend but this to me raises so major ref flags. If she is accusing you of cheating and trying to use this as proof then there is no trust in the relationship from her end. If you two have been together a long time and you think it's worth saving then I would suggest some counseling to work in her trust issues.

*red flags

it may not be lack of trust it could be self doubt or a lack in self confidence too that has mane feted as paranoia in the sense that he's cheating on her because she's not good enough. it happens to a lot of women even if they don't want to admit it.

pretty risky cheating on her with her mother. well go big or go home is what they say

ask her to call her mother and see if it was her who sent the flowers

I have to say that this is not okay. This is not normal behavior. She might need professional help.

I think it's kind of weird that you said it's ALSO her mothers name.

...and why did "Vanessa" send flowers to OP and not to both OP and the girlfriend?

That's also a good point.

Let her mom know what's happening. sometimes a little bit of shame talk from a mom is enough to make someone see the light

That's a very clever move to get a side chick with the same name as your girlfriend's mom.

Rookie mistake to tell her your address!

I can see why she is upset though. The flowers should have been wrote out to her and op. Another thing Venessa is a very popular name and most parents will refer to themselfs as mom or dad to their child's significant other. Or they might even adress themselfs with their last name. It's a FML for you because her mom caused so much grief but it is an FML for her as well.

No, there's no excuse for that. Even if she calls her "mom", she's got to know her own mother's name, and if she can't make the connection between her mother staying over and the flowers, she's either delusional or intentionally trying to cause shit.

???OMG. Did she realize her rather gigantic error. Or did she get stubborn and insist you are lying.

Sorry op, try telling your girlfriend to call her mom.