You might be playing a game with increment in time (also known as fischer time) with every move. Eg : 10|10 - This means that after playing every move you will get 10 secs extra on your clock and so will your opponent get.
Cheaters
I just played a 15|0 game against someone whose timer kept gaining about 60 seconds for every move he made (and mine none, obviously). I beat that cheater but that was one long boring game.
It could also be lag. Check out the FAQ on time/clock changes
Could anyone tell me how the time can be adjusted in blitz live chess when it only few seconds left. There are some players/cheaters who are not playing fairly by adjusting the time when he or she has few seconds remaining. Is this can not be corrected/controlled by chess.com system so game can be played equally? For those who are doing this kind of cheating, it is a shame on you.