Calling out at 35 weeks

I haven't used my pregnancy as an excuse for anything throughout my entire pregnancy. I called out one time, the day before my shower, simply because there was so much to do and we were overstaffed anyway. Well, I've been feeling really run down and overwhelmed this week, and was wanting to stay home to relax and regroup tomorrow (which is a quiet, slow day, I already checked to be sure). I just can't get rid of the guilty feeling. There are girls who call out and leave early over headaches, toothaches, etc... But me who's working full time right up until the end , is feeling guilty about staying home for one day. Any advice as to what to say to my manager about calling out? I don't want to use anything serious as an excuse, God forbid.