Feeling so rotten today!!

I'm 5 weeks pregnant and was sick for the 1st time thiss morning and ever since then, I've been SO tired all day, I can't stand up for long without feeling feint, I just wan to go to sleep but I can't cos I've got a 2 year old to look after (who's been SO good all day!!) And I just keep crying at nothing at all, just burst into tears!!