What to do if I feel that I am being discriminated against at work because I’m pregnant?

There is no simple answer, because it depends on the circumstances. An employer is not allowed to discriminate against an employee due to a disability and/or pregnancy, and may have to make "reasonable accomodations" to her condition. Reasonable accomodations are changes in how a job is done, or the provision of some assistive device or technology, to let the employee do her job. However, the employee must be able, with such accomodation, to do the job--if the employee cannot do the job, then the employer may suspend or terminate him or her, since they don't have to pay someone for not doing what they were hired to do.

Say that you are a cashier, and the only problem is you can't stand as much. Since you can be a cashier with a chair or stool, the employer would seem to have to accomodate you by providing such and therefore has to let you work.

On the other hand, say that your job also involves stocking the store, which entails lifting packages which could weigh 15 or 20 pounds--but that you can't do that right now. If that's the case, and there is no easy way around it (for example, you are often the only person in the store, so there is no one else who could do this for you), then they may be  entitled to not have you work until and unless you can lift that weight: that's because in this case, you  simply can't do the job.

So the answer to whether this is discrimination depends on the circumstances. You may wish to contact an employment law attorney, or your state's equal or civil rights division or agency, to discuss the matter.