Thight muscle sore/pain during pregnancy

Hi, I am 18weeks preggie. Weight & height are average. My thight muscles often feel very sore & pain after standing or walking more than 10mins. Gynae said its normal. I only feel ok (not in pain) when I sit or in sleeping position. I can't even walk to my neighborhood garden nor simple window shopping. Just wanna ask if anyone of you have the same problem too?