Severe cramp on my side when walking

This has been going on majority if my pregnancy but it seems to have gotten worse recently. I am 39 weeks exactly and when I try and go on a walk to get a little exercise I get a severe cramp on my lower right side. My placenta is toward my stomach (top) so I dont think it is directly related to that. I have to slow WAY down- and I was already walking slow! Like from a 20-24 minute mile to a 30 minute mile.

Anyone know what causes this or have a similar experience? Doctor isnt worried and just advised me to listen to my body (slow down if I need to, etc.)