OP here! The NHS site says:
"Serious side effects
It happens rarely (in less than 1 in 100 people), but some people may have serious side effects when taking sertraline.
Book an appointment with your doctor if you get:
changes in your periods, such as heavy bleeding, spotting or bleeding between periodsweight gain or weight loss without trying
Call a doctor or contact 111 now if:
you have feelings of overwhelming happiness (euphoria), excessive enthusiasm or excitement, or a feeling of restlessness that means you cannot sit or stand still
the whites of your eyes turn yellow, or your skin turns yellow (this may be less noticeable on brown or black skin) – these can be signs of liver problems
you cough up blood or have blood in your pee
you have black or red poo or blood in your vomit – these can be signs of bleeding from the gut
you are bleeding from the gums, or get bruises that appear without a reason or that get bigger"
So yes, I would contact your GP or 111. My GP sent me to A&E. They will want to check your platelet level & blood clotting. Mine was fine so they said the drug was likely to blame and to come off it straightaway, although the Dr I saw had never heard of the side effect & had to look it up! If the bruises have appeared since you started the drug, it's likely to be the cause so don't panic! They also found a trace of blood in my urine (another possible side effect), which they said was more likely to be because period due than the drug (period started next day). However, they subsequently sent me for a CT scan, I'm assuming as a precaution because of the urine although no- one has been able to explain why (the appointment was sent to me 2 days after I left A&E). I've not had the results yet and am too scared to call, although I'm hoping they would've called me by now if a problem?!
I only had 4 bruises when I went to A&E but they were weird purple ones that appeared and grew within hours. My GP called them Purpura. I've not had another purpura since that day but for sometime after would get the odd much smaller, paler bruise that would appear then disappear really quickly. Touch wood they've stopped appearing so hopefully the drug is now totally out of my system.
The doctor at the hospital said my GP should report my case to the "yellow card" scheme for the stats, so I will ask them to do so as I feel people should be aware.
Having health anxiety, the whole experience has traumatised me and I will never touch another antidepressant again.
Good luck and please update when you can.