My 1 Year Old Son Was At The Lake On

My 1 year old son was at the lake on Sunday, on the sandy/muddy beach playing and the following day he had red bumps that resemble bites behind his knees. They increased in number overnight, some on his arms, armpits and trunk. He is 21 lbs and healthy. I have been putting hydrocortisone 1% on the areas and applied calamine lotion tonight. We thought this may be chigger bites but he really wasn't in the plants or brush near the shore. He was primarily playing in the sand and mud. Should I take him to the pediatrician or continue treating as I am. How long till I should see improvement?