Could be staining, could also suggest caries (some teeth look grey/pitted).
Which tooth is causing pain? What kind of pain? How long for and does anything worsen it? The pain from a (failing) root filled tooth would be a constant dull ache/throb, may feel spongey and or painful to bite on and may be worse at night, when lying down. Your sleep would probably be disturbed and you’d want painkillers.
If you are pregnant then on the NHS everything IS ‘free’ (you’re exempt from charges). Do you have a local dental hospital? They usually offer free treatment, particularly if you go in with pain. Alternatively call out of hours/111 who can direct you to an emergency dentist (who will only be required to get you out of pain, not look at your whole mouth)
Options for root filled teeth if theres a hole could include a new filling or a crown although it’s not always possible. If the tooth is causing pain as a result of a failing root filling then realistically it would need to come out. Then options to replace gap could include bridge, denture or private implant but these options are dependent on other factors such as gum health and caries risk.
In the meantime use a fluoride toothpaste, interdental brushes and minimise frequency of meals/snacks/sugary drinks (including tea/coffee with sugar and fizzy drinks!) to 3/4 times per day. Not always easy i know.
@Tigerbus Sorry to say, that won’t do a lot.