I get a lot of upper back problems feels tense, can it be stress or my bra? I'm 100lbs and c size breast do I need a more supportive bra?

Good 1st option. Being a thin woman, the size c breasts could be causing a lot of strain on your back. Definitely start with getting some good, supportive bras. Try to identify any other triggers, such as posture, that could be causing the pain. If making some lifestyle changes doesn't help, your physician can help you identify and alleviate the pain.
Possibly. You may want to go to a store where they can do bra fittings & help you pick out bras that are most supportive for your size. Bra fittings are usually free. It is important to have the correct band size. Most of the support should be coming from that rather than the straps, which can worsen should problems if all of the support is coming from them.
Possibly. The use of a supporter bra would be a relatively easy first step. Other measures such as exercise, stretching, massage, stress relief… may also be helpful. If not, your family doctor may have better advice after direct examination. Best wishes.
Either/both. Start with wearing a more supportive bra. Give it a week or two and see if it relieves your pain.