Titchou is right, it could be a lot of things. If you are having difficulty breathing to the point that it's interfering with normal activities, you need to get it checked out right away. If you have a regular doctor, try to call him or her. If it gets real bad, go to the ER. Difficulty breathing is one of the things they are there for.
It could indeed be asthma, even if you've never had it before. I developed asthma for the first time just a few weeks ago. I was fine one day, just a little itchy from allergies. I woke up the next day with my chest so tight that just walking down the street made me pant like I was running flat-out! I went to an allergist, and he got me on some great meds.
Or, you might have some kind of infection in your chest. Allergies can cause postnasal drip, which can lead to infections.
Or, it could be something else entirely, that has nothing to do with allergies.