Starting Prozac for maybe 5th time. Feeling breathless

[panic_down_under]

You don't think starting and stopping it so much has changed the way it affects me

It may have had some effect on your brain. Your heart, unlikely, especially not on QTc.

and also being on Losartan with it?

There is no significant drug interaction between fluoxetine and losartan, nor is there evidence that losartan affects the QTc interval.

I'm only on 10mg of Prozac also.

Do you plan on increasing the dose to at least the recommended minimum dose of 20mg? If not you should as being on potentially sub therapeutic doses for extended periods increases the risk of ADs pooping-out.