need advice please!!!

Hi nannies,
I'm new to this group and to nannying. Ive been with my family for about 6 months now but have been working in the child care field for 4 years, at first i loved it and i get paid 15$ an hour and get plenty of sick and pto. About two weeks ago i started to feel very over worked and under appreciated. The mother works from home wich is hard cause i feel like she is always over my shoulder or running to the baby when he cries, the dad works FT and is barely ever home, I had a very good relationship with the mom and feel like its deteriorating. She has been asking me to work longer hours (i work 40 hours a week now) and is asking me to come in on my day off/ weekends to help, she is never rude about it but i can defiantly feel some tension/resentment if i say no. I have a very busy life outside of work so its hard for me to take time out of my personal life to be on call 24/7. I also get paid under the table which is really starting to bother me with tax season approaching. I agreed to this because i was desperately looking for a job and just didn't really think too much into it when i first started, but now im thinking its a bad idea and are wondering ways i can bring this up? I briefly mentioned it to her one time before and she told me that we could do it on the books but my pay would stay the same thus me getting paid less. I do a lot for this family, i use my own car to go to the grocery store for them, sometimes using my own money if im out and she forgot to give me her card. I try to make myself as available as possible but im really starting to get annoyed with the fact that im starting to feel over worked. I feel like if I do start getting paid on the books that my rate should increase . Any advice on these topics?????
Thank you in advance!