There's really no reason to be concerned about pregnancy. Approximately a zillion things can cause nausea, and pregnancy is just one of them. Given that you are using a highly effective method of contraception correctly and have not missed a withdrawal bleed (and in fact had one very recently), the odds are endlessly more likely that you are simply feeling nauseous for one of only a zillion reasons.

For what it's worth, a two-hour time variation absolutely does not meaningfully impact the efficacy of your pills on combination brands like Tri-Sprintec; taking your pills within two hours of the same time each day is actually very prompt, not "sloppy."

Long story short: don't sweat it. There's always some tiny, tiny, tiny chance of pregnancy while on HBC, but nausea alone is not remotely close to being a specific enough symptom to trigger that concern.