Had a baby 3 months ago, just got back on the treadmill and now I get headaches. Why is this and what can I do to continue to run?

Start slowly. This could be an exertional headache, which is brought on by physical activity. Take it easy and slowly build your level of activity to see if that helps. Staying hydrated also helps with headaches. See the doctor if the headaches don't get better, start getting worse, or are occurring more frequently. Keep a diary of the headaches and show it to the doctor.