Just wondering if swollen breasts that are a bit tender and have grown in size is a symptom of the injection. And what is the likeliness of getting pregnant on the shot if you have had them on time?

Immensely unlikely that you are pregnant its quite normal to gain weight and get larger breasts while having the shot.
My one bit of advice is you look up all the side effects of having the shot you really do need to know them all.

Depo-Provera