I own a lacrosse ball and after rolling a few times, it ended up being more painful because I'm forcing the stretch even more. I think it might just be an over-stretching and over-straining thing caused by calf muscle weakness.
Tennis ball stretches are only good for temporary relief but actually cause the issue to worsen. Plantar Fasciitis is a result of the Plantar becoming over stretched, it's supposed to be tight. Any calf exercise that helps increase the blood flow to your feet in the short term and increase circulation in the long term will help to heal the injury. Blood repairs all damaged soft tissue through the oxygen and nutrients within it. If you are having a flare up then what you want is a cold wrap for the temp pain relief rather than stretching which worsens it. If you really want to speed up the recovery get a blood flow stimulator, one that uses a gentle approach, this is the one I used:
Plantar Fasciitis Treatment by King Brand
I myself came down with PF some months ago. Before that I didn't know what it was, had heard the name but didn't think I would ever get it. Then one morning the bottom of my foot hurt.
I do weights too which tend to build up a lot of tension in my feet. I remember a few days before the PF happened that I got severe cramping in the bottom of my foot and I just sat back and felt the pain and didn't try to stretch the bottom of the foot out against the cramp. Then I got the Plantar Fasciitis. I scoured youtube for answers. Here's what works for me:
And notice I said works--I don't think you ever get over PF once you have it. It may go away but always comes back. I stretch out my foot pulling the toes back with one hand and pushing down on the heel with the other. Then I take the hand I'm using to press down and massage the bottom affected part lengthwise. Relax, and repeat over and over. Then I massage crosswise. That tissue on the bottom feels grainy and bumpy. I also use a heavy duty electric massage in which I use vibration on the bottom tissue while stretching it.
The cause of it? I don't know and I had a very hard time finding out why. It's just a part of the body that is breaking down and degenerating. I know you don't want to hear that but there it is.
good luck with that,
Fellow PF sufferer