Your emotional needs are not satisfied,
therefore vulnerability creeps in.
Even if there's another girl whom you regard highly with a weird
feeling, I don't think that would constitute you being 'cheap', as
long as you don't cheat on your gf, by going anywhere
further.
Contrary to popular belief, I do not believe in the abstinence of
evil for the preservation of good - I believe in knowing temptation
(evil) and get in touch with them because only when you have bathed
in pool of 'temptation' and gotten out 'unscathed' by choosing
good, would you actually gain enlightenment and self guard yourself
from future adversity.
Of course, the common man's belief is that if you cannot handle it,
avoid it - I would say that is hellva right too.
But I always choose the unorthodox way to manage myself.
Rule of thumb is: if you feel something is not quite right, always
look into your relationship for reasons and resolve it. Most of the
time, the actual reasons are not found beyond the boundaries of
your relationship (although it may seemed so). In simple: You may
think that it's the girl's appearance in your life, but I would
tell you that it's your relationship. 
Cheers