Is it my problem?

I had been with this engineering company for about 8 months now. My job position is administrative & marketing executive. My salary is about $2,200/month. The company is a small company run by a husband and wife team with 5 employees including myself.

A little bit about myself:-

1) I'm a fast learner
2) I complete designated tasks fast
3) I'm pretty good at multitasking
4) I perform better when there is a steady flow of tasks

Everything's fine and normal when I first joined. The boss did mention to me there's nothing much to do for my position. And yes I do like this job. There will be sporadic tasks to do every now and then, with stretches of free time in-between. At first, probably also because it's the probation period, I'd always ask if there's anything for me to do. And then as time goes by, I got confirmed for my position and got used to the fact that there're alot of days which I'm free, I don't even ask if there's anything for me to do anymore. If they have something, they will come to me. And trust, they rarely have much for me to do...

And my boss, he doesn't tell me when he's going on an overseas trip and much less leave a list of things for me to do when he's away. Take example for now, I'd been free 2 days in a row now.

But of coz' I have to find something to pass time during these free slots, I'd take out some files and pretend to be working. But in actual fact, I'm actually doing my own personal stuff. My conscience felt bad. I often felt like I'm being paid to do nothing and sometimes I just give up on pretending, pretending is bloody exhausting. By giving up I mean, not bothering to do the 'pretending' too much. I also felt the lady boss throwing glances towards my direction, you know like checking out if I'm actually working. I felt like pointing it out to her that her husband (which is the boss I report to) did not give me ANY thing at all to do. And maybe I think too much, but I can almost swear the rest of my colleagues give me funny looks as well. Coz' often they're the busy ones doing the engineering project and stuff while I just sit at my desk all day doing nothing.