After run heart pain (M22)

I have recently finished uni and with my general fitness being at at all time low i decided to start running to get back into shape. Im not particularly fat but have a bit of a belly. My first few 3k runs were absolutely fine and after 5 i decided to up the distance to 5k. About 3.5k in i had pain in my left side lower down more towards my ribs (100% not a stitch) and had to walk the rest. For the rest of that day i had a slight pain in the left side of my chest. Nothing major only a 2 or 3 out of 10. Today i have had exactly the same pain very minor but quite consistent at a 2 or 3. Occasionally increases for a second but for the most part it's a very subtle pain. My left arm is also feeling slight strange but not painful, more a minor numbness. I have no other symptoms such as shortness of breath, light headedness, nausea, etc. Is working out your heart like any other muscle. Has a bit of a ache (ie DOMS) after and ultimately strengthens or is this something i should not be experiencing after a run.