Should I talk to my boss about difficulty with tasks at work?

I am 18 weeks pregnant and a waitress. I recently started having trouble with certain tasks that come with the job. I can still do most of the job, but have been having trouble with things that require bending and/or lifting heavy items. We have had pregnant women working there during their entire pregnancy before so I would think my managers would already be aware of potential problems I might have. I didn't think I would have any problems with just asking my coworkers for help, but I seem to have been wrong. When I have asked my coworkers for help with tasks, some people have gladly helped and even gone so far as to stop me from doing things to do them themselves. However, I have had a few encounters where the coworker has said no to helping me or even once gave me a rude and sarcastic retort that I didn't deserve. I don't want to name names or cause any problems, but I think that sitting down with my boss and laying out what I'm having difficulty with may help in the future when things get even harder (which I'm sure they will). However, I don't wnat it to seem like I'm trying to get out of work, I'd gladly do other tasks in place of it. How should I approach my boss without seeming like I'm trying to take advantage of my pregnancy or should I just continue to ask others for help and avoid asking those who have given me problems in the past?