Similar issues for me as a teacher of sen - wasn't immune but ended up being off work from 9 till 21 weeks as it was strangely prevalent this year (never come across it in 11 years teaching, not immune, but thankfully didn't catch it - ended up having 5 tests in total) as it is notoriously difficult to both know who has it plus the infectious period is before the rash occurs. Lots of good info/ advice on the health protection agency website.
The waiting for results is a little frustrating, but remember you could be immune (some children and most adults show no symptoms or are unaware they'd had it) and if you are not but have caught it (the second test will show if you have or not) there will be careful monitoring of baby. I think there's only a 1 in 4 chance of it passing to baby if not immune and if you've caught it - however, for me probably the constant unknowns and ifs and waiting - and every single medical person having a completely different view about how seriously to take it at any point in pregnancy - was excruciatingly stressful.
Get the test NOW, wait to see what the result is and take it from there - post again on this thread in a few weeks if you find out you're not immune. If your collegue has a rash and been feeling ill, she may have already passed germs on, when rash shows they are no longer infectious.
This was why it was so hard at school - the kids aren't that poorly sometimes so are at school with a sniffle but may not get rash till about a 4-7 days later, and there was exactly 31 days between a couple of known confirmed cases in one class (rash) and then a couple more in the same class. It's a similar level of infectious rate as chicken pox, but must be taken seriously as the rash is similar to rubella and they need to rule it out (though should have had rubella immunity test at booking in).
Slapped cheek disease is not supposed to cause abnormalities (unlike rubella and measles) but there's a slightly increased risk of miscarriage if virus is passed to baby before 20 weeks (remember though it incubates for a while) or makes baby aneamic, worst case would fetal hydrops hence the scans, and I believe they would give a little blood transfusion in utero. In essence its just a matter of starting from the test and many women find they are immune.
Hope this helps - I found knowledge was power esp with different medical opinions and awareness (even my gps had to do their research about it, one was even a professor and found all the ins and outs a muddle!)
Take care xx