Can a employer fire you just after returning from medical leave?

The anwer is, "it depends":

1) You can't be fired for using Family and Medical Leave Act Leave, if you and your company, were both covered by FMLA: or for using a similar state leave law.

2) You can't be fired simply because you are disabled (if your condition qualifies as a disability).

3) You can't be fired if the company agreed to let you take leave and that was the reason you took the leave.

However:

4) Unless FMLA or a similar state law were involved, companies are not obligated to provide medical leave at all--which means they can terminate employees who miss work for medical reasons (unless the employee was using some accrued or earned PTO to cover the leave), either as soon as the employee goes out or afterwards.

Note that even if you qualified for and used FMLA leave, if you exceeded the 12 weeks you get per year, you could then be fired.

5) If you come back unable to do the job, even with reasonable accomodations, they can terminate you--employers don't need to retain employees who can't do their work.

6) If you lied to the employer about, or even did not keep the employer accurately informed as to, the time you needed off and the reason for, they could likely fire you for that reason.

7) You could be fired for wholly unrelated reasons (e.g. poor performance, violating company policy).

It's not clear what you mean by saying that "doctor/specialist time off" was used against you--you may wish to repost your question with more detail.