My period's been late before, it's scary. Chances are if the tests are saying you're not pregnant, you're not pregnant. It can often just be stress that's causing something like this. But go back to the doctor to talk about it, if in say a week or two, you still haven't had your period.
Now, I assume you've worked out that you ABSOLUTELY need to be taking some form of contraception. Nothing like a pregnancy scare to make you re-evaluate the importance of a condom and other forms of 'just not getting pregnant', eh?
If there's not going to be a condom involved when you have sex, then you ABSOLUTELY need to be taking other precautions as well. In fact, I'd say a condom AND another form of contraceptive are the best ways to ensure you don't get pregnant.
Whether that's an IUD or The Pill, then you need to do that. Though, with antibiotics there can be the chance that they make things like The Pill LESS effective. So you need to either not having intercourse (there are other things you can both do that mean you won't get pregnant but still have a good time) or make him wear a condom. If he doesn't want to, that's his right, just as it's your right to tell him if it's not on, it's not on.
You might be in the kind of relationship where condoms aren't a thing you use because you're not having sex with other people and neither of you have any plans to have sex with someone else. But, be aware that not using condoms comes at it's own special kind of price that does not in any way ensure you aren't susceptible to diseases. A baby, hopefully, grows up after 18 years and leaves. Herpes is forever.
If you end this relationship, then you now have the responsibility to get yourself checked for STIs and STDs before having sex with someone else. No matter how certain you are of one another's sexual history, by not using a condom, you're opening yourself up to a lot of diseases. Diseases that kill.
I don't mean all of that to scare you. But with everything we choose to do, especially in regards to our sexual health, we have a certain amount of responsibility. You can choose not to use a condom, but it comes at the price of a lot of different things, including but not limited to, pregnancy scares and STI checks.