Hi Stacey -
As you say, we've all probably got it. MRSA acts like many viruses in the sense we can have it and not "have" it, but it puts us at a greater risk of having something sooner or later. Since pneumonia is peculiar in its tendency to "save up" irritants and infectious agents in the lower lobes of the lungs, having had this infection puts you at a statistically somewhat greater risk of getting it again, but you're equally likely to not have that. You could have it show up in some other body system, but the lungs are very popular with MRSA anyway. You also may never have another active infection. It's an extremely random problem.
I think it would be very wise indeed to be seen by an IM doctor for bloodwork, and if positive you will need to do the whole nine yards this time, especially because you put yourself, given your overall history, at risk of other, secondary problems if you wind up hospitalized for another MRSA infection. The more you can stay out of the hospital (at least in the role of patient) the better for you.
I hope this is helpful and that everything turns out fine. Please update us, and follow up with us here as needed.
John Kenyon, EMT, CCT
Non-invasive cardiology tech, Emergency and Critical Care technician, Critical Incident Stress Mgmt. specialist