I agree pretty much with what Ken has suggested.
It may be that you are so tall that getting "under" the sail (your long arms put you quite a ways out away from the boom and sail) is preventing you from developing the necessary amount of Mast Foot Pressure (MFP).
Also, since you were on your first sesh for the season, you may have been moving back on the board a little too quickly, or perhaps placing your rear foot a little to far back before the board gets fully planing.
Try moving back a little more slowly and progressively, try the seat harness with the lower hook height, shorten your lines as much as you can (long arms again here), and try to stay a little more over the board until you are fully lit.
Hope this helps,