She's pulling away from me and becoming distant, what do I do?

This girl I've been dating for a few months has become distant lately (we're both 24) She never texts me first anymore and when we're together she looks like she has a million things on her mind. She's not been her usual happy self.
I do like her, she supported me through a rough patch with my family 3 months ago, when she could have walked away and now she's dealing with me going through a bad phase of depression. I'm changing meds to stronger ones and the side effects have hollowed me out this week. I get grumpy easily these days and I guess I don't really show much affection. She said to me last time that she felt she initiated most of the affection between us, which is true, but she wasn't trying to accuse me. She knows I can become distant without realizing. She wants to understand what depression is all about and I appreciate this all new to her. She's very patient and wants to be there for me, so I don't understand why she's being like this? We organised to do something this weekend, and yesterday she asked me if it was still happening (she's going on holiday next week) I said I wasn't sure because these new pills have made me feel really shit, and wouldn't be the best of company. She said she understood (we spoke on the phone) but I could tell she sounded disappointed.

Somebody please give an insight into what's going on here. Really confused!
She's pulling away from me and becoming distant, what do I do?
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