**They might act like this because it's not illegal**, and someone in HR had the bright idea to put pressure on employees who dare to take more than a day or two sick leave per year. They might be trying to reduce some internal cost metric associated with employee sick leave. 

Whatever the reason, while their actions may not be strictly speaking illegal, *they are obviously working in terms of making one think twice before taking sick leave.* They clearly have the company's and not your interests in mind. 

However, implying that you might be engaging in some activities that are against company policy without any evidence to support this may be bordering on harassment and so they might be in violation of their own policy regarding professional conduct and safe working environment. You might want to read up on the law regarding that and bring some talking points to the next HR meeting. 

I believe sick leave is a benefit which is part of your benefit package and that an employee should feel free to use, just like vacation leave and other benefits. Unused sick leave is often not paid out upon separation from the company, so you better use it. It is completely acceptable to not only use sick leave for physical illness but also for mental health -- a "mental health day" is not only healthy and good for you, but it is also good for your productivity and long-term effectiveness if it helps you destress, refresh, and come to work in a better disposition, with better motivation to do good work. 

Moreover, you shouldn't have to ever explain the reasons behind sick leave to anyone, *unless* the employer has a clear written policy to this effect (assuming it is legal for them to ask this -- sickness is part of your medical history which is protected by privacy laws). Otherwise, "I am not feeling well and taking a sick day" should be sufficient 99% of the time.

I encourage you to **educate yourself about your rights regarding sick leave** to which you are legally entitled, and on policies which HR might be violating by pressuring you and making accusatory statements, and to stand up for yourself in the next meeting. It does not sound like you are abusing the sick policy, you are merely using the benefit which is part of your benefits package, just like vacation and salary. Hopefully they will get the point and consider dealing with you more trouble than it's worth. Good luck!