There are a few signs that you may be overdoing it. Avoid exercising until you’ve reached exhaustion or breathlessness.
Don’t push yourself, listen to your body and stop when necessary. Be sure to rest when needed and keep a water bottle handy to stay hydrated.
Additionally, if you feel dizzy, stop walking and take break before heading home. Take your phone with you if you’re walking alone as well.
Especially during the second and third trimester, consult your doctor if you experience any of the following: vaginal bleeding, swelling in the calf or joints, joint pain, chest pains, or dizziness (source).