there are a couple points in your post that suggest something else you need to look into; FMLA leave. Regardless what PTO or vacation you had or didn't have, if FMLA was applicable and you had not exhausted your entire allowable time, they still could not fire you for missing the covered lost time. It appears you have an ongoing medical problem so it might be applicable (if the other qualifying requirements are also met)
Employees are eligible for leave if they have worked for their employer at least 12 months, at least 1,250 hours over the past 12 months, and work at a location where the company employs 50 or more employees within 75 miles.
and the reason for the time off must be a qualified reason as well. It must be a serious health condition as defined by the FMLA.
If you think it might apply, either research the topic or post back and it can be discussed further.