I need advice..

I'm a full time cosmetology student and we have actual clients every day we're open, Monday-Friday. I'm 26 weeks pregnant. On Friday we were super busy and I ended up being on my feet and working around color from 10-3 by the time I finished up my last client I had to rush and go sit down because I was feeling really off and my feet were killing me and I was seeing spots everywhere. I just thought maybe my sugar dropping because I didn't get to eat lunch. I'll ask my dr about that at my next app. But my question is, when should I start slowing down? When do I tell them I can't do certain services because I can't be on my feet that long?