Doc says i have prehypertension. 135/80. i'm 19, weigh 135 lbs. difficult to exercise because of asthma. should i be concerned?

Yes, be concerned: This blood pressure is not going to cause symptoms, but as you get older, it will get higher and will likely start to cause issues. You should focus on a low sodium, low fat diet and follow up with your physician. My primary concern is that if your asthma interferes with your ability to exercise, then it is poorly controlled and there are options for better long term control.

Maybe, maybe not: If you take your blood pressure 10 times a day you will get 10 different readings- BP constantly changes. There is no reason to be concerned about a single reading. Some asthma meds raise BP. 135/80 is normal but 135 is a bit higher than average. You can get a BP monitor for under $50 and do more checks. If often on the high side see http://tinyurl.com/6nr6ams for diet and lifestyle approaches.