Why has my milk supply decreased?

I exclusively breast feed my baby and in the beginning around the 3 month mark my breast had a ton of milk. I could get like eight ounces from 1 breast at 1 breast pumping session. Now I usually allow my baby to feed from my breast directly but I have noticed my boobs feel a lot emptier now and do not produce a large amount of milk anymore. My baby is 6 days from being 6 months and it seems as if my supply had dwindled. I cannot understand what is going on bc in the beginning my boobs were always full and now not so much. I did a little research and according to my findings it is possible that I have not been consuming as many calories as I should and in turn my body does not have enough fat to make milk. I also noticed that I have started to GAIN weight as oppose to losing it. What do I need to do to get my supply back up so I can feed my baby or do you think it is now a good time to start weaning him. I am trying to get back to my pre preg weight and have been trying to cut a few calories out