sleep troubles

hi everyone i have an 18 wk old lg who used to be a great sleeper but in the last 3 weeks her routine has gone mad and i cant seem to get her back she never had a good sleep during the she used to just have a few naps of around 45 min but she would go to bed around 10 and sleep till 7 but for the last few nights she seems to stir around 12 midnight and thats it unsettledfor the rest of the night it ois starting to wear me down i have a 6yr old also who needs me too im exhausted any suggestions on how to settle my little girl and get her back on track please