4 weeks ago I had anal sex the condom was ok (checked) but since a week I started to have a skin rash and now since two days I have flu like symptoms plus diarrhea, could it be HIV?

Not from last sex. You were protected & symptoms are not of any rec net sexual encounter. Rash is from something else. Hiv rashes occur at least months after contracting virus. You might be at risk if you had previous unprotected anal sexual encounters in the past.
Yes. Get checked. May be unrelated, or not. Other risky behavior in the same general time period?