I was just brushing my teeth and some of my gums started to bleed. i haven't brushed my teeth for a long time , is that the reason ?

Yes: If you have not brushed your teeth for a long time your gums will bleed due to increased build up. Just keep it up with it since you just started and add some flossing in to help keep things extra clean and proper. As the tissue gets cleaner it will eventually get healthier but if you have not had a dental exam and cleaning in a long time too I would suggest you do that soon as well.

Inflammation: Your gums are inflamed, and that's the reason they are bleeding. It's time to visit a dentist for a thorough professional cleaning. Afterwards, reintroduce yourself to twice daily brushing and flossing. It will help your overall health, your teeth, and even take care of bad breath.

Infection: Bleeding gums are infected gums. Probably caused or exacerbated by your poor oral hygiene. You need immediate professional care. See a a dentist to help you return your mouth to health and to help you keep it that way.