Hm, I've had what felt like a bruise on ribs before after various work outs. It's not, it's just a tight something or other. It's painful. But I have also had injuries. They are different. A pain is sharper, if you will verses a muscle strain. I tend to work through a muscle strain and rest with an injury. It's been a few days, is it feeling better?
I used to get my ribs messed up when sparring in martial arts, but it shouldn't happen with normal weight lifting. Abs can cause this, because they work this area especially if you're not doing them with the core in perfect posture, and who does that all the time? To me, from what you describe, it could be anything really -- you're 55 years old, and that means wear and tear adding up plus longer recovery time. Frankly, personal trainers range from wildly overeducated because they're dancers or physical therapists as well to ridiculously dangerous -- a lot of the exercises you seem them teaching are horrible for the lower back. I can tell you that when I got hit in the ribs or thrown, my chiropractor just found where the rib was and pushed it back in place and it stopped hurting. Only thing, actually, a chiropractor has ever been able to fix for me. I would ask, do you mix any cardio in with the weight lifting? Do you warm up before starting? Does any exercise you're doing feel like you're straining, especially in the area where you're hurting and when reaching overhead? As for the rest, my experience is that strained ribs don't take that long to heal. But everyone has different pain thresholds in different parts of their bodies. Also, swinging those kettle balls has always seemed like not only a worthless exercise to me, but a dangerous one as well, but an awful lot of people are doing them nowadays so it must be for some reason. Resting isn't usually a great option unless you're severely injured or chronically injured because it takes so long to get back to your fitness level, especially as you get older. Keeping moving in ways that don't hurt the area is probably better. Another problem is that you don't really know for certain it's the ribs --it could be something else in the same place or radiating from somewhere else, such as the shoulder. When inflammation or structural problems show their ugly heads it often impinges on a nerve, and that can send pain all over the darned place, which is my problem because of a bad neck and now probably hip problems. At 65, after a lifetime of basketball and running and martial arts and some weight lifting I'm in constant pain, some of it structural, some of it nerve, most of it mysterious. So take care not to stress injuries, know the difference between soreness and injury, and try to keep moving if you can in ways that don't affect adversely the injured area so you keep your fitness level and don't stiffen up.