Don't stretch as if its a tear in your muscle, it will have started to form scar tissue around it and stretching might rip it and slow down the healing process.
If its still painful, try ice and heat alternate.
Once the scar tissues has formed and is still soft (from 4 weeks after injury onwards....up to about 12 weeks), its best to have specific soft tissue massage on it to break down the healed tissue and re-align your muscle fibres to prevent any future niggles.
As for returning to pool.... well only you can tell. Depends on the severity of the injury. Listen to your body. If swimming hurts, don't do it. There's a degree to pain as well though. I would have advised maybe stay off from any sternous exercise until the scar tissue has formed, which is 4 weeks or more.
Meanwhile you can do other stuff that doesnt make it hurt as much.