The rashes can be allergic reaction
to the progesterone
or the SHERIC capsules which contains carbonyl iron as some patients do not tolerate carbonyl iron and do develop itching with rashes.
Rare cause can be some form of viral infection but less likely as there is no fever, and rarely due to altered liver
function tests which are common in pregnancy
but less likely at 16 weeks.
I would advice you to go for few blood tests like CBC with platelet count
and ESR,and Liver function test. Consult your doctor with the reports.
If the reports are normal and itching very severe she can try levocetrizine tablets at night to relieve the itching and find out with what is she allergic by stopping progesterone or the SHERIC tablets alternately after consulting your doctor.
Don't worry, take care.