Some variation in peak flow is normal. Take it when you get up, and in the early evening and plot the results as a graph. If your morning reading is a lot lower that suggests asthma, but modern treatment guidelines suggest that inhaled steroids are the drug of choice for most asthmatics rather than relieves, which is what Ventolin is.
However, if you only get symptoms after exercise then pre-treatment with a reliever might be all you need - if indeed you have asthma!
The Asthma UK website is very good, plus they have a phone number that will let you speak to an asthma nurse:
http://www.asthma.org.uk/