Am i lifting weights wrong?

[JohnJ]

im 16 and i used to lift weights last year and could only do 25 lbs with reps of 15 but now i cant even do 10 reps of 20 lbs. ive gotten really skinny and lost a lot of strength. Now when i do the reps with the 20 lb dumbbells (for bicep curls) my arms get really weak and tired and i cant really flex my bicep ( i feel the burn more in my forearms up to my elbow crease) What am i doing wrong?