Every time I try to play fast hammer on and pull offs on my bass with my 3rd finger, it just becomes tender and weak! I did this once in February and now its better so I figured trying it now wouldn't be bad. (I tried fast hammer ons on the 3rd to 5th frets of the G string and it immediately felt tender) and this time I tried it on the 7th to 9th fret and it happened again god damnit! I think it's not as bad this time but still annoying. How long will it take this time for the tenderness and weakness to go away!?
Hey Awesome,

It sounds like your finger muscles are still getting used to playing single line/lead type parts. 
I would suggest working with scales or try slower hammer-ons and gradually ramp up the speed.

With exercising your fingers like this, it shouldn't take more than 3 weeks to a month to gain more strength.

It could also be that you're doing the hammer-on/pull off too roughly or in an awkward way.
It's difficult to diagnose well without seeing/hearing it. If you could show a video, it would help us a lot with helping you diagnose the issue.
"Every time you pick up your guitar to play, play as if it's the last time" -Eric Clapton