Changes in hair, scalp

I've always had soft, shiny hair.  I've also had extremely greasy hair if it is not washed everyday.  Beginning about 2 months ago, my hair began to knot up really bad on the underside.  I've never, ever had this issue before (and I'm almost 28).  My hair also used to be really bad with oilyness.  Lately, I can go 2 or 3 days without washing my hair and it looks fine (except for all the knots.)  I don't know if this is a hormonal thing or what.  I haven't changed anything when it comes to my diet or hair products.  I actually spent less time this summer using heat products than I ever have.  I don't know what to do or if is something to worry about.  Any suggestions? Ideas?