Hetti, my dcs have always behaved much better with other people - nursery, school, grandparents, aupair even dh - than with me. The change is noticeable to my mum and dh.
It is a fact that they can more whingey, demanding, clingy with their mother. For example, ds has almost always struggled with me during nappy changes, such that sometimes I have to wrestle and pin him down. At nursery and with our aupair, he just lies there happily. So much so that my aupair changes his nappy once I am out of the house.
My dcs have always been like that with me, even from birth and throughout my year-long maternity leave. So I don't have any reason to attribute it to using a nursery.
I think it is a good thing. If your ds is (at the stage where he is) going to be difficult, would you not rather he was difficult with you at home than at nursery? He does it because he feels safe enough and confident of your unconditional love to explore negative feelings with you. Other times, they are cuddly and loving.
It is still a PITA though.