Your first time together! I hope you both feel like it went well. If so, wonderful. If not, let me reassure you that sex often gets better and can feel more fun, pleasurable, and connecting with time and practice. And practice can be pretty great together.
LARC Is No Lark
But about pregnancy risk…Your girlfriend has chosen to be on a highly effective long-term form of birth control called a LARC (that stands for long-acting, reversible contraceptive). Implants can provide years of protection against pregnancy and they are very effective because it’s implanted once inside a woman’s body by her healthcare provider rather than relying on someone taking a pill every day or getting a shot every few months or using a condom every single time. But speaking of condoms: if you have any concern about passing STIs (or just want extra peace of mind about pregnancy risk), then consider using a condom when you have sex.
The bottom line is that your girlfriend is on a highly effective method of birth control which makes it extremely unlikely that she’d become pregnant even if you ejaculated inside of her (which you didn’t anyway).
You Are Free to Explore
As you two explore your sex lives together, you might enjoy the SexEtc.org website or books like Moregasm, Sex Made Easy, or Becoming Orgasmic. Your girlfriend may already be orgasmic for all I know, but I mention this book because it takes many women some time before they learn to experience orgasm with a partner. Not everyone knows this, so I just like to mention it to people who are relatively new to sex in case you two want to explore orgasm possibilities together (without pressure, please; pressure is the enemy of orgasm!). In sum, I wish you both the best as you begin your sexual journey together and I hope these resources are helpful to you both.