As a messenger I get on and off my bike a lot and I would feel a twinge if I got on or off on the right side of the bike (my right foot was sore). I guess putting tension on the achilles tendon as 531Aussie mentioned. The riding itself wasn't a problem, I just had to be more careful walking (avoiding running and hard heel strike) and got on and off the left side of the bike. I never had to stop riding though I cut back on my hours working. I hope you get well soon!