Is my employer ripping me off?

I started work at a pub three nights a week. it all seemed to be going well, and my employer said " i pay cash, my rate is 5.50 an hour, don't don't worry i pay your tax" i gave her my n.i number and she says she clears it with her accountant. i've worked there 5 weeks now and have no statement of employment no contract nor any paperwork whatsoever and to make matters worse, i now think shes ripping me off- i worked there half six til 11 and got paid 17 quid! i do need this job, and the tax thing bothers me because i'm a student and i shouldnt be paying it ANYWAY. i like my job and my boss but what are the legalities of this and what should i do, everybody gets treated the same but it seems to be only me who is bothered!