Sketch lose refences

Hello to everyone,

I am pretty new in NX and I have problem with sketch. First my sketch was green, I've addeda dimension and unexpectedly becomes blue. You can see in the attached file. Then when I draw something, in one case NX gives me tangent constraint option and in other he doesn't. When I try to add constraint it lose references.

In second sketch, if I just add some circle sketch becomes overconstrained.

Can someone please explain me what is the problem?

Thanks in advance