I am pretty sure this one is used in some specific situation, say, SEX. According to this question, I think there is a need to get detailed explanation because it is difficult to understand. The meaning from the sentence itself is same to what LINDA said below( others' are incorrect). However the real meaning is " I am not an EASY person, but I could be as EASY as a really pervert/bastard".

可以是朋友之间的玩笑，有时与性有关，一般都是男性说的

I am not the kind of easy-going person. If you let me go easy, I am afraid I won't be that kind.

意思其实就是我平时都是很有原则的人，不是随便的人，但是某些时候我一但失去了自己的原则，就会什么事情都做得出来。。。。。

楼下的MM，偶不同意你滴说法，这人随便起来的话，那就不是用还要随便所能形容的了，应该称之为变态。 这句话等同于，我不是随便的人，但是，如果我随便起来，我就会很变态。 这样理解好象更准确是不是。毕竟变态不是一件很容易的事。哈哈