In reply to Jack McKechnie:
I bruised or cracked my sternum many years ago while playing a game and end up smacking into someone running at full speed as they were running towards me. It took a good number of weeks to get better. I wasn't climbing at the time.
If yours is better after only 2 days I wouldn't worry too much. It doesn't sound that bad. Avoid doing stuff that makes worse, in particular the exercise that caused it, and I imagine you'll be fine. If what you do makes the pain worse for more than 24 hours later you'll probably need to back off a bit.
Post edited at 21:59