Rio, I love my husband's story (he's an electrical engineer) of starting his electro magnetic fields and waves course (1964ish :-) and the professor is going on and on and everyone was clueless and my husband raised his hand and admitted cluelessness and the problem was the students hadn't had vector calculus yet. So the professor stopped and taught them vector calculus first :-)

E-mail, stay after, visit, whatever! Admit you're lost. If nothing else it will get the lecturer to know who you are and that you care. When I was taking social psych I was also taking statistics from the same professor and it was only that that got me a passing grade in statistics! He "recognized" me so "liked" me.