broken rib question

Yesterday I was out on a local trail. My right foot connected with a stump and I started to fall forward. As my left foot went forward (sort of in a lunge position) I leaned back and threw my arms up to keep my balance. At that point I felt a pop in my ribs and a sharp pain. After stopping and walking for a minute I resumed my run. Later in the afternoon the pain was worse so I went to the doctor. An x ray showed the second rib from the bottom on the left side was broken. Has anyone heard of a rib breaking without actually falling or running into something? The doctor didn't seem too concerned so is it ok to continue some easy running? I'm 40 years old and this is my first broken bone.

I'm born at odeni-gida town,odeni-gida district Nasarawa state of Nigeria. Thank you baba for what you are doing Mallam Agah Okonu.The osu of odeni-gida district.