Premature contractions I think. Experienced that with my 2nd pregnancy for being too active and walking too much. and almost gave birth at 31wks cos of the contractions which caused my cervix to open at 3cm then. Got meds in hosp and my baby held on til 35wks.naturally waterbag leak and she came out 2.3kg healthy. Phew. Pls rest but if u stil feel pain, do consult doctor.
Could be some premature contractions provoked by the long walk. Don’t walk so long as you’re already 2nd trimester, and do more bed rest for next few days and you should be fine. However if the ache persist then pls do update your gynae