If she is nicer when you are alone I'd say it is because in public view she is making sure that people do not think that you might be her boyfriend.
1	If	if	SCONJ	IN	_	4	mark	4:mark	_
2	she	she	PRON	PRP	Case=Nom|Gender=Fem|Number=Sing|Person=3|PronType=Prs	4	nsubj	4:nsubj	_
3	is	be	AUX	VBZ	Mood=Ind|Number=Sing|Person=3|Tense=Pres|VerbForm=Fin	4	cop	4:cop	_
4	nicer	nice	ADJ	JJR	Degree=Cmp	11	advcl	11:advcl:if	_
5	when	when	ADV	WRB	PronType=Int	8	advmod	8:advmod	_
6	you	you	PRON	PRP	Case=Nom|Person=2|PronType=Prs	8	nsubj	8:nsubj	_
7	are	be	AUX	VBP	Mood=Ind|Number=Sing|Person=2|Tense=Pres|VerbForm=Fin	8	cop	8:cop	_
8	alone	alone	ADJ	JJ	Degree=Pos	4	advcl	4:advcl:when	_
9-10	I'd	_	_	_	_	_	_	_	_
9	I	I	PRON	PRP	Case=Nom|Number=Sing|Person=1|PronType=Prs	11	nsubj	11:nsubj	_
10	'd	would	AUX	MD	VerbForm=Fin	11	aux	11:aux	_
11	say	say	VERB	VB	VerbForm=Inf	0	root	0:root	_
12	it	it	PRON	PRP	Case=Nom|Gender=Neut|Number=Sing|Person=3|PronType=Prs	20	nsubj:outer	20:nsubj:outer	_
13	is	be	AUX	VBZ	Mood=Ind|Number=Sing|Person=3|Tense=Pres|VerbForm=Fin	20	cop	20:cop	_
14	because	because	SCONJ	IN	_	20	mark	20:mark	_
15	in	in	ADP	IN	_	17	case	17:case	_
16	public	public	ADJ	JJ	Degree=Pos	17	amod	17:amod	_
17	view	view	NOUN	NN	Number=Sing	20	obl	20:obl:in	_
18	she	she	PRON	PRP	Case=Nom|Gender=Fem|Number=Sing|Person=3|PronType=Prs	20	nsubj	20:nsubj|21:nsubj:xsubj	_
19	is	be	AUX	VBZ	Mood=Ind|Number=Sing|Person=3|Tense=Pres|VerbForm=Fin	20	aux	20:aux	_
20	making	make	VERB	VBG	Tense=Pres|VerbForm=Part	11	ccomp	11:ccomp	_
21	sure	sure	ADJ	JJ	Degree=Pos	20	xcomp	20:xcomp	_
22	that	that	SCONJ	IN	_	26	mark	26:mark	_
23	people	people	NOUN	NNS	Number=Plur	26	nsubj	26:nsubj	_
24	do	do	AUX	VBP	Mood=Ind|Number=Plur|Person=3|Tense=Pres|VerbForm=Fin	26	aux	26:aux	_
25	not	not	PART	RB	_	26	advmod	26:advmod	_
26	think	think	VERB	VB	VerbForm=Inf	21	ccomp	21:ccomp	_
27	that	that	SCONJ	IN	_	32	mark	32:mark	_
28	you	you	PRON	PRP	Case=Nom|Person=2|PronType=Prs	32	nsubj	32:nsubj	_
29	might	might	AUX	MD	VerbForm=Fin	32	aux	32:aux	_
30	be	be	AUX	VB	VerbForm=Inf	32	cop	32:cop	_
31	her	her	PRON	PRP$	Case=Gen|Gender=Fem|Number=Sing|Person=3|Poss=Yes|PronType=Prs	32	nmod:poss	32:nmod:poss	_
32	boyfriend	boyfriend	NOUN	NN	Number=Sing	26	ccomp	26:ccomp	SpaceAfter=No
33	.	.	PUNCT	.	_	11	punct	11:punct	_
