An at will employee can be fired for any non-discrimination reason.
Your post mentions no gender, race or age based discrimination. So, the question is whether you have a medical basis.
FMLA rules only apply to an employer that employs 50 or more employees within 75 miles of the worksite. And, I am not sure your non-work injury would implicate FMLA anyway. Too little detail exists in your post to know for sure.