Please tell me before I ring in sick - could I get fired in this situation?

[yellowboka]

I work in housekeeping/cleaning, it's quite a physically demanding job.
I've only worked there for 3 weeks.
I'm 14 weeks pregnant although haven't told them yet as I was worried they'd turn me away due to the nature of the job and lifting etc.
They're short-staffed as it is, it will cause a really difficult day if I call in sick but I'm really, really nauseous and light-headed and almost passing out - I'm sure it's because my iron is so low.

I absolutely can't lose this job. Can I get fired for calling in sick 3 weeks in or because I'm pregnant and it's an unsuitable job or what?

You don't have to tell until you hand in your matb1 at around 6 months . However the employer cannot do risk assessments (in this case such as lifting, chemicals etc) or allow time off for antenatal appointments unless they know.