To start (since I know this will likely impact the response I receive) I am in my mid 20s, moved back in with my parents a few months ago after hitting a bit of a hard patch.

I've noticed lately that interactions between my parents have changed. I assumed it was part of work stress, finances, etc. Tonight I walked over to the computer where my mother was sitting to grab something and noticed she was talking to someone she worked with. I managed to read some of the conversation and saw them discussing where they would meet tomorrow ("movies then go to the room") and calling each other "babe". I am all over the place emotionally right now, to the point where I am sick. I have no idea what to do. I feel like I need to confront my mother not only because I'm angry but out of sheer fear of what my father would do if he found out.

Wow! You saw a lot with one short peek -- that this person works with her and is male (?), and you were even able to read some of the IMs.

Your first obligation is to your mother, to tell her what you saw/found. She needs to know. (Your father may be well aware of this, by the way.)

Well I definitely went back over to ask her something to see a bit more.. guess I left that out.

I confronted her, she said it was "nothing" and had some crazy story for everything I saw. "The movie and a room" was them and friends. They both apparently call everyone babe (mind you, I have never ever heard my mom call anyone babe other than my father). It turned into this massive argument but hopefully this will make her slightly more aware of her actions.. or one can hope.

You are not your parents' moral compass. Having been married for many years, I'm guessing your dad is well aware if anything fishy is going on. Plus, maybe just maybe, your mom's explanation is dead on, and you are worried about nothing, since she didn't seem concerned about your looking over her shoulder and wasn't minimizing the screen every time you got close. It might all be perfectly innocent.

There's a fair possibility that you are reading too much into an innocent situation here. I have various groups of friends and a certain way of addressing one another does seem to evolve in some groups. I hardly ever call my husband darling for example, so it's not a term you would hear me use much, however I have one female friend who uses the term profusely so I often find myself using it to her in return. There are friends I call hun, love (although that's a rather well accepted term locally here for even relative strangers), babe etc. There are also often working situations where particular terms become commonplace that may not be used elsewhere.

We can alll become slightly different versions of ourselves in different situations. I mean most of us have the 'professional persona' that we use in certain work settings but we would't use the same persona at home with our loved ones. I remember feeling quite surprised, and impressed, to hear my daughter's professional tone when she was dealing with some work issues from home, being more used to the 'daughter tone' that she uses with me lol.

Similarly a bunch of women together often speak and act quite differently amongst themselves than they do when men or their family are around.

You caught your mum in a mode you are not familiar with and from one overseen message have jumped to the conclusion that she is cheating on your dad. That's really not enough evidence by far.