Darkbrown is old blood and nothing to worry about - a heavy persistant fresh bright red bleed is always a concern, but the older it is , or the longer it has been in there, the darker it gets.
It could simply be that your cervix was blocked or bunged up , maybe due to a clot or a polyp or swelling or irritation or cervical erosion problems.
Whatever the cause the old menstrual lining either didn't shed off and escape the vagina during a period when it should and is taking its time shedding, or if it did shed like it was supposed to, it is struggling at times to escape the uterus and the longer it takes to get out the less oxygen it has left to show bright red and it turns brown and sludgey. It is normal as you get older anyway that periods become more brown sludge than bright red, just due to aging changes which no one tells you about in school.
It isn't a fresh bleed - so while it is your period blood, it is prevented from escaping when it is fresh for whatever reason.
And as you walk about in your day to day activities a bit more manages to escape the cervix which makes you think you are still bleeding - when in reality you are not. This is the last period still working its way down.
It is annoying and doesn't look great but it is not dangerous.
It will happen if you had stopped your periods for any length of time on meds or the pill for example and the lining has been in there longer than the month...even a late period would be darker than an earlier one in terms of the age of the menstrual lining.
Just think when you have a nose bleed - what colour is the stained hankie, pillow or clothes? - it turns brown.
A cut on the finger or a scab on a wound - bleeds bright red initially then scabs over brown as the scab blood or stain loses oxygen and bright redness. Even a soiled sanitary towel that was bright red - will turn brown in the bin as it waits to be disposed of.
You can see you GP if you feel really worried and they might run tests or prescribe blood thinners to you - but be warned you you could end up with a gusher of a period on blood thinners as they don't clot and it could last a long time with fresh runny bright red blood so it probably isn't worth bothering about.
It may be that in a few weeks any clot in the way will have cleared itself out in the next period or two and you'll return to a more normal routine period.
For some ladies it just persists like that. You can douche (wash out) the vagina if you want to try and clean things up inside - but that can disrupt the natural PH in the vagina and may lead to thrush.
Some ladies find that after a D and C procedure which cleans out the uterus lining will also return them to a more normal period experience too.
On balance I would leave it be, and see what happens in the coming months, and i suspect your GP would advise the same thing. Pant liners were invented for this and other discharges mid cycle.
My periods started on the browns in my early 30s - and stayed like that with irregular periods for years and years, till I got the mirena installed aged 43 and none since that kicked in.
Stopping your periods with mirena coil for up to 5 years would spare you any monthly embarrassment from the slow moving menstrual flow turning into brown sludge.