How should I explain this?? Question about resume….this is long….sorry!?

Hi everyone! Okay so here is the deal. I have been applying for some positions, but I don't think the reason I am getting any calls back is because of my resume. So I was a bank teller for 8 months at one bank, was offered a position with better hours and 4 dollar pay increase at another bank so I accpeted that position. Which was fine, my boss at the first bank understood and said that she would have done the same exact thing. Okay so I was at the position for 5 months and then my mother fell ill. She was in the hospital, and had to have someone with her 24 hrs a day when she did come home. I quit my job there, in order to care for her. In February, I was able to re-enter the workforce and found a position in the dental field. I LOVED the place that I worked, and got along super great with everyone…besides one person (more on that later). Okay so Iast month, after 9 months of being there, had an AWESOME review that same month, was going to get a raise in December. I got extremely sick (kidney problems)…I was out for a week. I was on major pain pills, I couldn't even drive to work. My doctor even wrote a note and faxed it to my boss. So I get back and I go in that following Monday and I was "let go". My boss was crying while doing this, which made me angry. She kept saying that she was sorry and she didn't want to do this (if she didn't want to why did she!?) So of course I come home. Later that night, I get a phone call from one of the girls I worked with who was super close to me. She informs me that my boss told everyone that she let me go. Well you know how offices are, and gossip…come to find out, a co-worker (who happened to be my bosses sister) didn't care for me and took this time that I was ill to bash me and claim that I didn't do my work….which caused me to get fired. this wasn't the first time either! She did it to 2 other employees that she didn't care for! (I guess I should have seen it coming, but my boss liked me so much and I had amazing reviews that I thought it would never come down to this) While being fired, my boss said that if things ever changed in the office, that I could come back?!. Well, I don't want to go back…I just want to find a new job, but I have a feeling that I am not getting any calls back due to the fact that I have had so many jobs in such a short time. I am a hard worker! And I do my job, just seems to be bad luck! Also, I work a job part time (cheer and dance coach) that I have been with for over 2 years, so it isn't like I am a job hopper!(But I don't put this on my resume, should I?) Please someone help, what do I do? I am young…22…and I am in the process of finishing college(I take online classes so I can work full time) So these are my only job experiences….Should I explain anything in my cover letter??