Question

is amplitude of an shm always +ve?

Answer 1

What does a negative amplitude mean?
You could have one, I suppose, very easily mathematically:
y=-cos(wt)
and the negative part would just flip all the y-values, which in this case would be equivalent to a phase shift of pi.
But amplitude in SHM usually refers to the size of the objects oscillation, which can't be negative. So it depends what you mean exactly.