I feel your pain.
I'm going to assume that you've known your mom your whole life, and so you probably have a pretty good idea of how she will react when you eventually tell her the truth, and that your nervousness is justified. In light of this, I'd like to offer my sympathy, and my hope that if you do decide to share the truth with your mom, it all goes well.
I'm 30, living with my VERY Christian parents right now to save money while I'm back in school. When I didn't come home one night, it eventually came out that I had been at a woman's place for some fairly casual sex. My mom told me I disgusted her. My dad told me they weren't pleased with my life choices and we would talk about it later. That night I changed my facebook status to "looks like I'm moving again!" Then a message from my dad appeared in my inbox: "I saw your message about moving on Facebook. I think I might have an idea what this is about. I hope we can talk before you make any decision. Dad." I replied with I'm considering options but nothing that's a clear winner over staying as of yet. If you want me to stay there are two ways we can go. One is don't ask don't tell, the other is openness without judgement. I'd prefer the latter, as it's the only way we can have a relationship capable of growing, but it would be the harder of the two, at least for you and mom.
They haven't said anything about it since... which basically means they're not ready to have a real relationship with me. It's a little sad, but at least we have a platform of respect to live from.
...Kinda sucks when someone tattletales on you though.
My parents know now that I have different values than they do, that I respect theirs, and I want them to respect mine, but that they might not agree with some of my decisions. If they really wants to know about any part of my life, they can ask, but they shouldn't expect to hear what they want.
Hope this helps. Good luck!