Your hair sounds like it could have gone into a short anagen cycle, which I had. It's a disorder of the hair follicle, with various different possible causes (including hormonal). It's easily treated with low-dose oral minoxidil.
Anagen is the growth phase of hair, and for people with a normal hair cycle, every hair stays at this stage for 4-7 years (90% of their total hairs will be in this stage at any particular time). Eventually, each of these hairs will get a signal to slow down its growth (the catagen stage) and about 6 weeks later will move into the telogen stage, stop growing and eventually fall out. By this time, the new replacement anagen hair is in the follicle ready for another 4-7 years of healthy growth before it too starts to slow down, then die and shed.
When the anagen stage is shortened, each new hair gets the signal to go into the catagen stage prematurely (in some cases, immediately). Too many hairs are then in the catagen/telogen stages at once (instead of the 10% of hair that people with normal cycles have at these stages). When the telogen hairs die and shed, the disrupted cycle starts again, with the replacement anagen hair never getting a chance to grow for long enough before it too gets the signal to go into the catagen stage and start to die.
I take 2.5mg oral minoxidil every day to keep my hair in the anagen phase for the normal length of time. It took 4 months to start noticing the difference, but it's really amazing. After two years of taking it, my hair is about twice as thick, over twice as long and so shiny and healthy. I'm 55, post-menopausal, and I've now got the best hair I've had at any point in my life.
A dermatologist/trichologist prescribed my oral minoxidil - it's a private prescription, as it's officially an 'off-label' low-dose use of the drug, which was developed for use in much higher doses to treat high blood pressure. It's a cheap medicine though, and the cost of my 6 month supply is the same as an NHS prescription. It's been used to treat hair growth disorders successfully in young children and is a very safe drug, in the low doses required. It is now also available to buy from online pharmacies (after you've answered all the doctor questions to make sure it's safe to take with any pre-existing conditions or other regular medication). So you could try it for six months or to see if it works for you, without spending lots of money on a trichology consultation first. Oral minoxidil can increase hair growth in other areas of the face and body, as an unwanted side effect for a minority of people taking it. Interestingly though, in all the trial results of low-dose minoxidil for hair growth disorders, the unlucky participants who did experience the side effect of growing excess hair in other areas still chose to continue taking it. They felt its advantages hugely outweighed an annoying but relatively minor disadvantage - fairly easily remedied with the various different hair removal methods available. I have been lucky not to have experienced this myself, apart from it making my eyelashes much longer, which I'm very happy about :)