No method is 100% fool proof, and from what you said it sounds very unlikely.
But if your girl is worrying then chances are shes read up what the most typical syigns of pregnancy are and her mind is playing tricks on her and exibiting them.
So many people on here have xperianced signs of being pregnant because they wirher wanted to be pregnant so badly or they were so worried that every thing that they experianced they scrtinised and twisted it in to a potential symotom.
The spotting could have been (if she was a virgin) the remainder of the hymen breaking but if she was pregnant, for implantation bleeding thats a little early.
If you and her have only just started havigsex thats gonba mess her about a little anyway, most girls when they first have sex will miss a period or it will be delayed becuase of the changes and nervs.. And stress.
I would just chill muh amigo.
And good boy for using condoms... They are your friends!!