Sacral dimple in baby. Constipation.

My baby is 3 years old... She was having sacral dimple near buttocks from birth(pic Attached)... No doc seemed to be concerned Abt that ..said it is normal as we r seeing its base... By my repeated queries one of the pediatrician said to go for USG to be doubt free...( attached USG report also)...USG was done wen she was 16 months... Showing everything is normal....but I have few concerns which I want to ask u... my daughter always shows her right knee...saying it is paining...that pain is too severe... She cries profusely entire night... sometimes during day as well... Done USG of knee that is normal... Still she shows that knee...and has pain once or twice a week... My query is can this knee pain be related to that sacral dimple...did i miss anything... Can USG spine done at 16 months be wrong... Im always thinking may be she is having some issue with that sacral dimple there...that is why it is paining in her knee...to add here she is also having chronic constipation...plzz help me