fired after medical leave

Did you use paid time off for the leave? (E.g. if you had enough vacation, sick, etc. days to cover the absence and used them to do so?) And/or is the companay covered by FMLA leave (e.g. it has at least 50 employees within a 75-mile radius), you are eligible for FMLA leave (you have worked there at least a year, and worked 1,250 hours in the last year), and you *in advance* (if it was not an emergency; or if it was a medical emergency, as soon after initial hospitilization and treatment as possible) properly properly applied to use FMLA leave? Of if neither of the above, did the employer voluntarily approve your taking leave of this duration (preferably in writing, so you can prove the approval)?
If the answer to one or more of the above questions is yes, then should not have been replaced by another employee: the employer can't fire you for using PTO you earned/accrued; can't replace you for using FMLA leave to which you are entitled; and cannot tell you that you can take a certain amount of leave then, when you take them at their word, replace you. If this happened, speak with an employment law attorney: you may have a viable lawsuit.
But if the answer to the three questions above are all "no," then they could terminate you, even if you medically needed to be away from work. Regardless of your personal need, unless you used PTO or FMLA, or had advance permission from the employer, the absences were unauthorized and the law does not make employers retain employees who miss work like that.
Or if the replacement had--and can be proven to have--been in the works prior to your leave (for example, you were aware that due to performance or personality issues, or changing the job/position, you were going to be replaced), then they could legally continue with had been previously planned but which had only been interrupted by the leave; using even approved (e.g. FMLA) leave or PTO does not mean that you can't be terminated for unrelated reasons, just that you would not be replaced while on leave.