car push

I recently bought a 2000 Mazda Protege. The mileage on it is around 41500. A couple of days back while driving, my car shook like someone braked and I was pushed in front but after 2 seconds everything seemed normal and the car was rolling smoothly again. I was concerned at that moment, but forgot. But again yesterday similar (after about 3 weeks from the first incident) situation occured and I am worried that something is wrong with the brakes or something else. I happens while driving and I'm given a hard push like someone brake my car even though my feet is on the gas paddle. Anyone has any information on that, I'll really appreciate. Please help!!

I wanted to get the 30K service done on my vehicle, but the dealer of mazda gave me a price quote of around $480 :disappoin .
I'm currently a student living in Charlottesville, VA and if anyone could suggest me a good car service shop, I'lll really appreciate that.

Thanks in advance.