Oil alarm

[steplift20]

I'm mad, went down to go fishing. And this happened
Started her up,perfect
Drove her around ,no problems
Then after about a hour. I started her and the oil alarm
Went off.
Shut her down started her. Oil alarm went off ,so I took her back on land and and loosened the oil cap on the motor and it was pumping it into the oil resivoure ,so no issue with the pump
I did replace the cap/sending unit about three years ago.
What advice do you have for me? I'm going to go to my boat this week and check the cap/sending unit. After that I'm lost. Thanks for any replys