Concerning that toddler sleeps 18 hours consecutively?

My girl has always been a good sleeper, but lately she'll sleep all 12 hours overnight, then have a cup of milk and go back to sleep for another 5-6 hours. This has been going on for a month.

She used to sleep 8 hours, then take 3 naps during the day, but now it's like she just takes all her naps at once after her overnight sleep.

Should I contact her doctor, or just count my blessings while it lasts? She's acting normal when she's awake, and she's not difficult to wake up. (Accidentally did that a couple times. oops.)