Hi, I am new to bicycle and I just got a low-end Iron Horse mtn bike for commuting and occassional off road riding. The commuting is about 5 miles a day. I have had the bike for about a week. Today i ride to work after having the bike sitting at home for its first weekend, i feel that my butt was going up and down which didnt happen last week when i was riding it. And I am not going thru some rough surface, just pavement. I am wondering if this can happen if the back tire is underinflated. If thats the case, what can cause a tire to be underinflated after 5 days of commuting (which adds up to be about 25miles)?