shouldn't I enjoy parenthood more enthusiastically?

I don't know what's wrong with me. Shouldn't I enjoy the weekend with my kids...my family? Instead I am edgy (trying to hide that almost successfully) and annoyed by the fact that I can't just break out and do whatever I want to do. I really could use a trip to the fitness center right about now but there is no way I will get that time before nightfall...and I mean real night..like past 10 pm kinda thing. And that also only if I ignore the rest of the stuff that needs to be done in the house (laundry, cleaning, etc.). As I am writing this entry my 18-mo. old (Nayla) is hanging on my knee craving full attention. If I don't give her 100% of the latter she is just as cranky as I feel. Am I a bad mother? Am I just overworked and have not enough balance in my life? I don't know. The fact that I feel physically impaired (chronic f*in backpain, and strange stomach aches ever since pregnancy/childbirth) isn't helping either. ok gotta go. Nayla is now crying. sigh.