Hi. Try not to worry. In all sincerity, women (myself included) can have odd cycles. Hormones can shift even slightly and it can happen even if it hasn't happened to you before. Other things that lead to an 'off' or irregular cycle are stress, illness, changes in routine, weight gain or loss, etc. Even if it has never happened before, it can still happen out of nowhere.
You used a condom which is great. Those CAN fail but you almost always know if not always when a condom has failed. This involves typically a big rip and a big mess. Condom failure is why condoms have small risk of pregnancy associated with them. But if the condom you used was intact after intercourse, I'd not be concerned about this. And, most women get a positive test 2.5 weeks after pregnancy if they conceive. You tested negative at 2 weeks which is a really good sign you aren't pregnant. To be sure, wait a full week and after taking that test and take another which makes 3 weeks post sex.
Most likely, your hormones are acting up. Happens to the best of us. Hang in there.