Could be a combination of many things. A decrease in performace along with the higher heart rate even sounds like possible over training. You may need a rest to let your body sort itself out and repair.
Trust me - allergies can cause violent coughing - I've been there, bent over double coughing until I think I'm going to puke or my eyeballs are going to fall out and I have absolutely no asthma - doc said I have the best lung function he's seen in a long time - the coughing was all from the post nasal drip.