Problem while running

Question
I work out regularly, Cardio and Weight training.

Since a week or so, I am trying to extend my cardio time from 20 minutes to 40 minutes. To prevent myself from draining out all the energy, I am doing 20mins Cardio > Weight > 20 minutes Cardio.

But towards the end of the second 20 minutes session on the treadmill I feel an intense pain in the lower right chest (not always). Usually at my second last or last rib.

The pain usually goes away if I take a few deep breaths stretch myself a little. Sometimes I keep working in spite of the pain, but sometime I need to quit. The pain goes off 5 minutes after I quit.

I am 33, male. Otherwise healthy. Lifting happily.

Any ideas, what could be wrong? Shall I visit a doc.

Answer
Great job on working so hard Jack. This is kind of a tough one. I would feel terrible if I said don't go to see a doc and it turns out something is seriously wrong. However, it is likely nothing serious.  Pay attention to you gut instincts and if you feel like something may be wrong go get it check out.

That being said try to slow down during the second run and see if that relieves the pain. If it does relieve the pain continue to run faster, but stay below the pain. Often pain like that comes to tell your body you're working harder than normal and "I want you to stop!" It likely isn't anything serious, but that doesn't mean it isn't.

Let me know if there's anything else I can help with or if you have a follow up question.