I have been training Krav maga for about 4 months now, no previous martial arts experience and very little sport/fitness experience.
In that time, I got prolonged rib pain (3-4 weeks) on three occasions. The first time I went to a physician, who told me it's bruised but not broken, and just to wait, afterwards I didn't go to a physician. The second and third time, it hurt not only at the place where I got injured, but also at another, roughly symmetrical place in the right half of the ribcage (all three injuries were at the left half, but different heights).
We are a considerate group, and attack with rather too little aggression than too much. None of the injuries resulted from a sudden strong blow, which I would have assumed is a usual risk in martial arts. The first time I held a punching shield wrong (didn't brace it) while a partner punched, the second time it was a bear hug, the third time I supported a partner while rolling on the floor because she was about to hit her head, and happened to support her knee with my ribs (it was a slow movement, not a blow). Others in the group did the same exercises, including receiving many more bear hugs (I stopped after the painful one, which was at the beginning), and nobody seemed to have any problems.
It doesn't seem to me that the force in the injuries is too strong. I have the feeling (maybe I'm wrong?) that a similar force to, say, the thigh, would only leave a blue spot, if anything at all. Certainly not prolonged pain. And on the ribs, there is no visible injury afterwards, no blue spots. Just the pain.
Am I doing something wrong, because I don't know correct safety practices? Or is my ribcage unusually prone to injury, should I try to get a medical examination? Or is this normal when starting with such an aggressive sport? Or just bad luck? Should I be worried, and how can I reduce these injuries?