I'm sure it isn't a problem, since it is going off when warmed up brake lines get slow in cold, if you are noticing braking probs take it to a shop, if you are worried take it to a shop. Personally I would want to have a competent mechanic take a look when such an important component of my vehicle is acting up. I'm sure it has been cold before. How long has it been since you have serviced your brakes??