not to scare you, and it's very unlikely she would have an std and you would catch an std but oral with out and with protection is considered pretty safe.
1) With out protection you have an unlikely risk of NGU,Gon, syphallis, hpv, hepatitis, and hsv1 and rarer 2. Out of all those NGU would be the most likely.
With protection HPV, and herpes could be possible. Being that you are under 26 I'm sure you have your hpv vaccination which protects against not all but some of the more troublesome strands
Keep in mind the chances of her having any of them is slim, and the chances of transmission would also be slim from her to you. From you to her would be a higher probability but still slim.