Im getting a pain right under my knee when im running?

Yesterday was the second day I ran and it doesn't sound like much of an accomplishment but it really is for my lazy fatt butt. Now I'm not sure if its a bruise or if its because its hard for my chicken legs to hold up my heavy upper body. You might say I should loweer the speed but its only on speed five and I don't want to change how I'm doing things. What do you think it is or what I should do?