Can you get heatstroke or heat exhaustion from a 20-min. walk in 90+ degree temperatures? i took a short walk, and didn't think i was dehydrated, but started feeling headachy into the next day, but with no other symptoms?

It does depend on your physical condition and your tolerance for heat/humidity with activity as well as how strenuous the activity is. If you are going to be out in the heat/humidity walking, you should have a water bottle with you and continue to drink throughout your walk. Also, limit your time outside and slow your speed of walking. These steps should help prevent this from happening again.