feeling bad about not wanting to work til the end. any advice helps

I was originally planning to work until I went into labor but the bigger baby gets the worse my back pain is. I'm a manager in retail and I feel bad for wanting to go out early and I've already adjusted my schedule to have less hours but it's still dreadful every time I'm at work. I'm debating on starting leave at 36 weeks. I'll be 30 weeks on Saturday. 
The back and hip pain added to this new pressure I feel in my pelvis is terrible. Especially walking around. I'm a ftm and don't really know what to expect