Is it normal for my milk supply to drop after starting baby on solids?

Your milk supply will naturally decrease as your baby starts solids. What's happening is that your baby is slowly replacing your milk with other foods. If you want the weaning process to be slower I suggest you offer solid foods after the breast, rather than before. The best policy is simply to respond to your baby's cues, then everything should be in balance. If you feel like your milk supply has dropped and you don't have enough milk to satisfy your baby, then more frequent nursing sessions or pumping your milk should bring it back up.