Britney Spears and her mother, Lynne Spears, reunited this weekend and appear to be on the road to reconciliation ... and despite their recent problems, mom seems to realize -- her daughter needs her right now.

Sources with direct knowledge tell TMZ that Lynne's drop-in for Brit's birthday party Friday night was no coincidence ... but it is indeed meaningful, especially after everything the two of them -- the Spears family, at large -- have experienced over the past few years.

More recently, Britney torched her immediate family members in her memoir, 'The Woman in Me' ... and as it pertains to Lynne, she accused her throwing out precious valuables of hers -- something Lynne denied.

There were also reports of Lynne selling off some of Britney's famous clothes ... but she denied that too. Of course, Britney has also suggested her mom was just as culpable for her conservatorship as her father, Jamie ... and there's been bad blood for a while there.

The two of them attempted to bury the hatchet earlier this year, pre-memoir, with Lynne flying in to L.A. to meet up with Britney and her then-husband Sam Asghari ... but it was a brief chat, and it wasn't clear whether they were on the greatest of terms at that point.

Now, we're told Lynne and Britney are definitely in a better place ... not only did Lynne swing by for the birthday bash -- but our sources say they were 2 peas in a pod last night, which then turned into Lynne spending the night ... and also, hanging out with Brit today.

As far as what, exactly, spurred this latest coming-together ... our sources tell us Britney's incredibly isolated and alone right now -- especially after divorcing Sam. We're told she has almost no friends or confidants -- and it appears Lynne recognized that.

Britney is still close with her brother, Bryan -- who was also in attendance for her party Friday night -- but she doesn't see him as often. Now, there's another person in her orbit ... somebody of her own flesh and blood, no less.

It's great to see them acting like a family again ... but the same can't be said about Britney and her dad/sister, whom she has not been seen with for several years. By all accounts, she doesn't talk to them ... although there have been signs of warming toward Jamie Lynne.