It is wayyyy too soon to tell. No symptoms would be present yet. The headaches could be from the stress of worrying about it. Stress will affect you physically. Or it could be a pre-period headache. Tell her not to focus so much on a simple headache, that can be due to many things.
As for not wanting to take a test.....Tell her she has to grow up for a second. Not facing the truth is childish and will only increase her chances of harming the baby. If she is pregnant, pre-natal care if a definite, so there is no excuse to hold off on testing. Tell her whether she likes it or not, she needs to get over her fear.
Also, make sure you nicely suggest using protection next time. She obviously does not want to be pregnant. Tell her to stop acting like an adult if she cannot accept adult consequences. She is too young for sex and too young to have babies.
Just continue to be her friend, it's all you can do. You can offer advice but her decisions are entirely up to her.