Braxton hicks after walk

I got some fairly strong Braxton hicks after walking for 30 minutes today.  I haven't been very active this pregnancy because of severe nausea.  I am 24 weeks now and trying to do some more walking to keep at the weight gain recommended by my doctor.  Is this normal to have these contractions after light exercise, or is it a sign that I shouldn't be walking?  FTM and not sure what to expect.