Stellaria sp?

I believe it's Diodia virginiana, Virginia buttonweed.

If it is a Diodia, then your tiny pink flower would have to be ... maybe the base of a seed capsule whose top has fallen off? And your unopened flower buds would be less mature seed capsules. (It's trying to take over my front lawn, but I haven't studied its anatomy in detail.)

In any case, it seems unlikely there would be unopened buds and nearly-mature seed capsules cheek-by-jowl on the same part of the stem. You could go back and dissect those parts of the plant, to see what's inside.

Nice photos here:
http://www.cas.vanderbilt.edu/bioimages/species/divi3.htm

Also here, but unfortunately not of the seed capsule:
www.florabymax.net/ FLORAbyMAX/ Family-pdfs/ FLORA-P-Z/ Rubiaceae%20(Madder%20Family).pdf

Doesn't seem to be Stellaria, because those are petiolate. Not Diodia virginiana, because that usually has two sepals. Not D. teres either (lacks those bristles under the flowers and fruit). Those pink flowers and fruits remind me of Rotala ramosior. I wonder if this is Rotala indica? That species is prostrate, has 2-valved capsules, and leaves of similar size. Anybody familiar with that Asian import?

Think these are bloomed out, as missingtheobvious said, and that the true ID is likely Oldenlandia boscii. It seems to be a full match.