SUBREDDIT: r/relationships
TITLE: Me [25M] with my GF [26 F] of 2 years, Not sure how to say no to her without getting a bad response
POST: Occasionally I will have to say no to plans or sometimes cancel plans, it happens and I figure it's something that most people don't take to heart. Well my GF seems to.

Example from today which pretty much sums up how these instances go: Yesterday I asked if she would like to visit the cinema tonight but she said she couldn't as she was having friends over for dinner. Fine. 

She then asks if I would like to join them, I say sure and everything is dandy. Today I am feeling rough, I have backache (again, it started after sleeping at hers earlier in the week) and my head hurts and I don't really feel like hanging out in her uncomfortable flat with these mates (who I do like but they aren't exactly great company, she complains about them all the time). So I explain that I am not feeling too good and think I might take a rain check on this evening.

She replies rather bluntly saying fine but she was really looking forwards to seeing me. I say sorry, can we do something on Saturday instead (I am busy tomorrow eve). She says no and makes a couple of excuses that don't really mean we can't hang out. 

This happens more often that I am cool with. If I can't make a certain date she gets pissy and pretty much punishes me by refusing any alternative plans I make. She also lays the guilt on a bit by saying how she wanted to see me, I don'd mind this but I am not convinced this is true - she didn't invite me in the first place, the plans didn't involve me etc.

I think I could probably handle these situations better but how exactly is not clear, any ideas? I don't like this dynamic AT ALL, it stresses me out. 

I should note that I see her pretty regularly throughout the week, have hung out with her her mates a couple of times recently and all went well - I don't think I am inattentive in these areas.
TL;DR: 
GF gets annoyed if I am busy or have to cancel plans, gets a bit guilt trippy and moody about other plans - how do I deal with this?