DON'T FREEDIVE AFTER SCUBA!!
If you want to do both free and scuba dives, do them in that order. Freedive first, than have your scuba dives.
That way you'll be safe. If you did scuba dive, wait for at least 12 hours before you go freediving (or 24 if you went really deep / worked hard / cold water)
The problem is the fast ascent rates when freediving (60 - 90 meters per minute) and the recompression at depth.
When you make your slow way back to the surface at the end of scuba dive, you let the micro Nitrogen bubbles safely escape from your body. If you freedive, the bubbles can get compressed again and go into parts of your body where they will just fit. During your rocket ascend to the surface these bubbles will expand at such a rate that they can not be safely removed from your tissue and might block something, causing DCS.
So always wait 12 hours before freediving after scuba. Just like flying after scuba.