If this has happened suddenly then get yourself checked out ASAP.
Don't put it off - it might be something serious. For example ... The first sign I had of life-threatening blood clots in my lungs was getting out of breath immediately at the start of one of my forum rides in 2012. One day I was fine, the next I was short of breath after a couple of minutes of cycling.
If you don't improve and your GP doesn't suspect blood clots (the first one I saw didn't) then bring the subject up yourself. If necessary, you could go to A&E - they can do a quick and simple blood test called a D-dimer which is very good at spotting the signs of clotting.