Lots of good advice – bike fit is a likely cause, even nudging the tip of the saddle down a fraction could make all the difference.
To check the seat height it would be worth getting someone to ride behind you to see if your hips are rocking from side to side, if so it is likely your saddle is too high; another sign of a saddle being too high is if your shoulders are lurching from side to side. A few mm can make all the difference.