Coughing up black some when smoking marijuana?

i've been smoking weed for about 2 years, cigarattes for 6. i dont smoke weed regularly like i do cigarettes but ill smoke every other day or 3 days. within the past year or less i have been coughing up black flehm. it usually happens after i smoke, but sometimes it can go on and off for weeks even when im not smoking weed. i have to exhale until i hear crackling in my lungs and then it comes out when i cough. also if i try to eat anything after i smoke, i can actually hear it going down in my lungs. why does this happen to me? my brother smokes even more than me and he's okay and alot of my friends too. im 19 im 5'2 and i weight 101 pounds