Im 20 years old i was just diagnosed with a pars defect of the s1 and l5 i had surgery to get it fixed but now the pain is way worse why is that?

Sometimes the : Surgery does not heal in terms of the defect healing if a direct repair or if it did heal, possibly you needed a fusion of that level. Even with a successful surgery, there can be about a 10% failure rate in a nonsmoker and a significantly higher failure rate in a smoker. You need to see your surgeon but it takes months for a surgery like this to heal. Surgeon case assess your fusion status.