Can my employer tell me I cant use my PTO since Im pregnant?

As a general matter, you cannot treat an employee differently because she is pregant--that is illegal discrimination. (It is considered discrimination against women, since only women get pregnant.) So an employee could not be told to not use PTO simply because she is pregnant.  If you believe the employer did this--is not letting you use PTO for no reason other than your pregnancy--contact the federal EEOC or your state's equal/civil rights agency about fling a complaint.
An employer does have the right to manage workflow and schedule and does have  a right to deny PTO when the timing is bad for business. If you have already told the employer you will be taking maternity leave in the near future (e.g. under the Family and Medical Leave Act, or FMLA, or a similar state law) or using PTO for maternity leave in next few months, the employer could deny you the use of other PTO now, on the grounds that it cannot afford to have you out of the office so  often within such a relatively short period of time. In this instance, it is not denying PTO because you are pregnant--it is denying you the use of PTO when you have already scheduled other time away from work. This would be legal.