Well I may not be the best help but I have an office job and worked till the end.
It is safe to work all the way through unless your dr says otherwise. I am 38+5 and my last day is going to be next monday. I would have just worked until i have her but they needed a date. But with being a cna i know that lifting those people is a big strain on you but if your dr is saying it is ok and you are still doing it this far along then you should be fine. As far as the cramping goes that is normal. But if you are concerned at all then do not be embarressed to talk to him about it, that is their job.
Where about do you stay cause in the uk you can finish up to 11 weeks before your due date. I worked as a junior care manager for a home care company, i managed the carers who went to clients houseS and it was a really stressful job, if we couldn't cover a call we had to go do it ourselves. I was going to work til i was 36,1 but got signed off by Dr a 33,4 as i was finding it too difficult, i would just speak to your Dr and say that you're finding it hard to cope now with the manual handling, long hours on your feet. He should recommend you finish up early x