So I started Yoga about a week ago and I have a little pain in my neck and upper back and I find myself stiffening up. I don’t want to strain my neck and back more than necessary, should I give myself a rest for a few days? or is this just my muscles reacting to some new exercise?
Welcome.
Please outline the practice you have been doing and paint a verbal picture of who you are. The feedback for a man of 70 is different than for a woman of 19.
I’m a 22 year old female, before doing Yoga I didn’t exercise much, maybe an hour on the exercise bike here and there. Thanks for the help 
Okay let me amend my answer accordingly.
When a person uses muscles that have been dormant (unused or not used much) then it is to be expected that those muscles will be “sore” for about two days after use.
When a person is poorly instructed in poses (asana) there is a risk of soreness, overstretching, compression, and injury.
In the first instance, it is best to let the muscles repair themselves and only use them gently for the first few days until they do. Meaning use other muscles. If your practice allows for that diversity all the better. If your practice is fixed (meaning “the same each time”) then you’d likely be best served stopping until repair.
In the second instance it best serves the student to either find another teacher, another practice, another method, or accept the myriad of lessons delivered to human beings through injury.
gordon
I agree with Gordon and think that every yoga student should revise the exercises themselves, when they feel the teacher pushes them too far. That is why the exercises should be practiced with the proper breathing and concentration. When you feel that either the breathing is too heavy or you are unable to concentrate because of a pain, then don’t push it too far. The aim of the exercises is not to make them perfect from the first time, but to enjoy practicing.
It is ok to say to the teacher, this is too heavy for me, is there an easier variation of this exercise for me? The yoga postures have many levels of difficulty, and most of them have many variations - from easy to advanced.