If i had enlarged armpit lymph nodes would i be able to feel them? i'm very very slim?

If pathological, enlarged lymph nodes are obvious. I say it that way because an examiner can palpate normal lymph nodes with experience, especially in patients like you with a lean body habitus. You are at an age where you notice many things about your body you never paid attention to before, which can feed into anxiety about your health. If you feel ok, you are. Don't look for signs you're not.

You would be able to feel enlarged nodes in the armpit. You may feel the nodes every couple of weeks, if any of the lymph glands doubles in size, becomes painful, ulcerates, or other nodes become enlarged, or you develop fever, or lose weight, you should see a doctor. Wish you good health!