Should it become generalized and affecting all of the gingival tissues then it should be checked by a Professional to ensure there is no evidence of any other systemic disorder
Please see your dentist.
1- You irritated you gum by brushing harder than usual!
2- You have not seen your dentist regularly (every 6 months) and plaque is accumulating there that causes gum irritation and thereafter gum bleeding!
3- You are pregnant and due to hormone changes you are experiencing gum bleeding!
4- You have started a new medicine that one of its side effect is gum bleeding!
5- You had something hot, or scratched your gum by something sharp (like chips or hard cracker).
Please contact your dentist if you think none of the above could be the reason.