This is the first time I've been fired and aside from mixed feelings of anger and relief, I'm concerned about how this might effect my future chances. The dreaded question, "why did you leave your last position?" is causing me anxiety.
Prior to my taking this job I had been out of work for over a year due to medical reasons. I could not resume my previous job duties as they were too physical for me at that time and could not secure an 'inside' position. So I resigned. If I omit this last job from my resume there will be a huge gap in my employment.
I was on my new job for six months and got sick. I was out for two weeks (bad timing) and was under medical care. My company forced me to take short term disability. At the time I should have been off probation but because of a company merger everyone was "too busy" and it was postponed. Short term disability + still on probation = discharge.
I was not given any warnings and the person from HR said not to worry.
I applied for and I'm receiving unemployment.
How should I answer that question? By nature I'm too honest for my own good, but I figure prospective employers will find out I was fired.
How should I proceed in my new job search?
Sorry for the length of my saga and thanks for your advice.

Explain that you had to go on short term disability before you finished your probation and were terminated due to corporate policy.

If you were clearly fired from your job and not laid off because of a retrenchment-type process then it is best to be open and honest in giving the reasons for your dismissal.
It is also likely that this will be explored in a reference check, so the situation should not be misrepresented in the job interview.