Are my severance check deductions incorrect?

I was terminated April 28, a Tuesday and the second day of a pay period. I was offerred 4 weeks severance and told I'd be paid the 16 hours worked the 27 and 28. I was also told accrued vacation would be paid (20 hrs). This would all be paid on the next pay cycle,may 15th. I was also told my insurance coverage would end 2 days later on the 30th. I received the stub today and noticed they deducted insurance payments when the period was 4/27-5/9 and I was taxed at a much higher rate than my last pay check. 19.5% federal vs 11% as previous. A total of 35% vs 25% from previous. All other tax deductions were about the same as a regular check. My question is: was Health insurance unfairly deducted? Was I taxed incorrectly since they combined severance with regular hours worked? Any info you can provide would be great!