I just ran up a 3 blocks city hill, and I am having so much trouble breathing, why am I so out of shape?

Uphill running. Uphill running is always more taxing than flat running as you are moving your mass against gravity and so more energy and muscle burn is required. Always a good idea to incorporate hills into one's runs to build up this type of aerobic/anaerobic endurance.
Change it! Running hills is a challenge to anyone's aerobic capacity but heed the wake up call. Get back into shape by starting a fitness program with a mix of aerobic and resistance exercises. Start at an intensity and volume that is appropriate for you. May i suggest run/walking. Start by running for 1 min/walking for 4 for 20 min. Chunks. Then, increased the running until you are running the entire time.